Your security couldn't be affected to the point that you could lose your bitcoin. The online wallet would need the private keys - the signing keys to be able to spend UTXOs. But it's not going to have that. It's only going to have public keys for address generation and for watching the wallet and its balance updates.

Worst case scenario, your privacy is affected.

I understand that. It is important to distinguish closed-source and open-source wallets and their firmware. Open-source wallets have been thoroughly tested. Now, that doesn't mean that someone couldn't find a serious bug one day, but the same is true for Bitcoin Core. 

What kind of user errors are you looking to prevent by having Bitcoin Core select the UTXOs instead of you? If you already have a cold wallet on an airgapped computer, then leave it as your signing machine. You need to synchronize your online wallet. That's where you should manually select the coins you want to spend and bring them over for signing using the private keys in your cold wallet.

Working with cold storage like that isn't as easy as using a light client. But I think what mocacinno and myself have recommended is more user-friendly than adding additional steps to the process.

https://learnmeabitcoin.com/ and https://www.lopp.net/bitcoin-information.html will have a lot of courses you can go through and focus on what you think is important. Some are for beginners, others are more technical in nature and require that you understand the basics.

Bitcointalk isn't really a teaching ground in the sense that there are A, B, C... types of threads that you should read to acquire knowledge on a specific subject. People come here with genuine problems and concerns. So, you collect pieces of information here and there based on the comments of other users. That gives you a clearer picture.

But if you want a complete course on a topic, check out the sites I recommended in my opening paragraph. You can always ask questions to make sure if you understood something properly. 

Electrum provides the user with a lot of freedom on how they want to spend their coins. You can always see which of your UTXOs are confirmed and which are still in unconfirmed status. Using coin control, you select what you want based on your criteria. By freezing certain UTXOs, you won't be able to use them for upcoming payments until you unfreeze them. You can also label each of your transactions however you want. That can be beneficial for future transactions.

Pošalji mi ovdje u PM. Možda nekad i zatreba. Ako je cijena prihvatljiva, zašto da ne. Ako imaš iskustva s nekim od njih ili znaš ljude koji su bili zadovoljni ponuđenim uslugama, napiši mi. Nisam nikad volio Telegram i ne eksperimentišem puno sa altovima pa me i nema na njemu. Zaostajem za modernom kripto klijentelom. I onda se sjetim da je BitGoba odlučio da napusti forum zbog moje i mislim slackoviceve priče o shitcoinima. O živote nesrećo.  

The easiest way to do it would be by scanning a QR code for hardware wallets and signing devices that support that option. Can you imagine having to enter a private key character by character using something like a Ledger, Trezor, or the Jade?! But in this day and age when all respectable wallets are HD wallets, there is no need for such a functionality.

Correct. The only wallet I know of that understands Electrum's seed standard is Blue Wallet. However, that's a software wallet, not a hardware wallet.
 
The problem with that method is that Electrum doesn't allow you to import individual private keys or master keys either. That's true for many other popular hardware wallet brands as well.
Now when I mentioned it, I have never done any research if airgapped HWs like Passport, Seedsigner, or the Jade support the import of private keys.

Even if you uninstall Electrum from your computer, it's not going to remove the wallet files found under \AppData\Roaming\Electrum\wallets. They will remain where they are.

Your issue has been solved, but do you mind if I asked you what kind of backups you have for this wallet and any other for that matter? I am not asking you to tell me I keep one seed at my place, the other two at my sister's and parent's house, I am just checking if you have backups of your seed words and how you did that? Or do you perhaps only keep copies of your wallet files?

I think that notification could be improved from Electrum's side. "The address won't be recovered automatically", ok. But then they say "you may need to add it manually." To me, that sounds like the only solution is to import the private keys of the missing address. That's something a user wouldn't have because the default gap limit doesn't allow him to see such addresses. Perhaps "you may need to add it manually or increase the gap limit" would be a more complete version. 

It's a bit difficult to follow everything you have written, but let's try.

When you connect your Ledger to Electrum, you have to enter the correct derivation path for Electrum to know where to look for your transactions. Ledger Live calls them bitcoin accounts. Do you have multiple bitcoin account in your Ledger Live? If so, go to the one you sent the coins from in Ledger Live, click the gear icon to open the info window, and copy the correct derivation path from there. It should look something like this: m/44'/0'/0'. Ledger Live will display more digits, but don't copy all of them. Only the first two digits following the "m/44" part. You should then see your entire transaction history in Electrum.

If the unconfirmed transactions are there, right click on them and increase the fee. Use https://mempool.space/ as a reference for the type of fee you need for a slower/quicker transaction. The reason why your unconfirmed transactions haven't been dropped is that there are still nodes on the network that have them in their mem pools. Unless you connect to one that no longer has them to "cancel" your transactions, they will still be available until all nodes drop them.

Could you post a screenshot to show us the other problem where you said that Ledger Live keeps sending the same transaction over and over again? Cross out all identifiable and sensitive data you don't want the public to see.

I have heard that many times. A computer that you disconnect from the Internet doesn't become a cold wallet just like that. It's like saying I am a virgin because I only have sex on Saturday. It doesn't work like that. There are also people who refer to mobile phones as cold wallets with a claim that they keep their phones in airplane mode and turn their WiFi off. When they want to make a transaction, they turn everything on. This is very far from the truth.

Although I don't like limiting users and preventing them from using a piece of software how they want to, I like this new feature. It's very rare that an ordinary user will need to generate over 20 addresses at once. And as soon as one of those first 20 deposit addresses gets funded, Electrum will make another batch of 20 available. This change can help prevent newbies from believing they have the wrong seed if they have funded addresses beyond the default gap limit. 

He is using it on an airgapped device (hopefully). Electrum can be used either as a hot or a cold wallet.

A watch-only wallet has no private keys and can't sign transactions. The cold wallet needs private keys, otherwise there is no use for it. The online wallet is the one without private keys, created by importing (master) public keys that belong to the cold wallet.

No, it's the same passphrase across all of his wallets. Not that it matters. Memorizing seeds and passphrases is a bad idea. A knock to the head, the normal aging process, and a bunch of other things can make your forget what's important.

True, but again it's very unlikely that anyone, not just a newbie, would generate 20 addresses, not use any of them, and decide to use the 21st or the 31st. Perhaps there are cases where a bug in Electrum would do something like that automatically. That would then restore an empty wallet by default. Do you remember such an incident on Bitcointalk or have you seen it somewhere outside of the forum?

Since the ultimate goal is to move the coins to Trezor, I wouldn't add a third software into the mix. You have Bitcoin Core on an offline wallet and you set up another one for transaction broadcasting - that should be enough. Or the faster and easier way would be to just import the private keys from your old Bitcoin Core wallet into Electrum, and then send the BTC to your Trezor. Again, you could have taken advantage of Electrum's cold storage capabilities for greater security.

It's theoretically possible, but I don't see a beginner increasing the default gap limit, unless someone else set up his wallet for him and had access to his computer. To increase the default gap limit you would have to enter a command in the console tab. I think that's the only way to do it, and that's why I think it's a small probability that this is what happened.

That's correct. If you go to Coinkite's store, and scroll all the way to the bottom, you will see the Glow-in-the-Dark model all the way to the right. The product description explains that it looks white/transparent under normal light. The name tells you that it glows in the dark. The third version from the top is the regular blue-case Coldcard.

One of two things probably happened:
1. You ordered the wrong one.
2. They shipped the wrong one.

I know that you said that you installed Electrum "again", but it's still worth asking: Was the original wallet created on Electrum or some other software?
During the recent restoration process, did Electrum ever ask you for a derivation path or type of addresses you want to scan? That could be a sign that it isn't an Electrum seed.

Are you sure that you didn't create a BIP-39 seed instead of an Electrum-native seed?
The passphrase you have must be entered exactly as you entered it back when you created your wallet. That includes capital/small letters and even empty spaces.

If the restored wallet shows no transaction history, then you have a wrong wallet that you never funded. Can you remember if you or someone using your computer ever created a second wallet for whatever purpose?

Try to compare the addresses you funded in the past to what you get when you restore your Electrum wallet from seed+passphrase. Did you send money to it from a different wallet, an exchange or maybe a friend did it? Find one of the addresses the coins came from and see if you can find the same one under Addresses in Electrum.   

It would also help if you bought yourself a pair of special sunglasses that make everything around you look blue.
I am pretty sure they sent you the one with the usual transparent case by mistake or on purpose. You are right when you said that this is the model that appears in most of the YouTube videos, but I managed to find one featuring a pink Coldcard Mk4. I doubt the person in the video uses some special light effects to turn the device pink.

https://youtu.be/IJbog_k5XWE?t=932
You will see it at around 15:32 into the video.

Upravo to. Hardverski novčanik je dodatna zaštita tvojih privatnih ključeva ali nikako kompletan sigurnosni sustav koji je neprobojan pa prst u uhu i šta te briga. Ako ga neko ukrade ili ga izgubiš, iskoristi to vrijeme da u miru napraviš novi novčanik (po mogućnosti na drugom hardverskom walletu) i prebaciš svoj kripto tamo. Dok oni probijaju zaštitu (pod pretpostavkom da mogu), ti si svoj BTC već prebacio na sigurno. Ako ti znaju PIN ili seed onda ti naravno nema pomoći osim ako nemaš dodatni passphrase koji je lopovima nepoznat.