I have a lot of Sats and have used cold wallets but didn't like them so I put everything into a hot wallet (Coinomi) which is very easy to use with both my desktop and a mobile app and seed phrase recovery (which I like), but I do worry about long term security.
Let me be the first to tell you this is a TERRIBLE idea to replace cold wallets by a hot wallet! Long-term security shouldn't be your main concern, you should worry about short-term security first. I'm using Coinomi too, but only for small amounts.
If you don't like your current cold wallet setup, buy a hardware wallet (Ledger or Trezor). Switching to a hot wallet is indeed very convenient, until everything's gone all of a sudden.
I suppose I would need to save the .dat file to another computer in case my node fails or the computer running the node crashes.
Another computer wouldn't be my first choice. Several different offline media (microSD, USB-stick, burn a CD, .....) give you better odds. Test your backups, and make a new backup once in a while.
Do I need to protect the .dat file in the same way I would protect a seed phrase? Can someone with the file gain access to my coins? If my box crashes will I have to download the whole Core again in order to recover my wallet with the .dat file? The Core tutorials I look at are either too technical or don't cover things simply enough for newbs to understand.
You can set a password in Bitcoin Core. Don't lose it.
The wallet.dat is all you need to backup, the 300+ GB blockchain can always be downloaded again when needed.
2. I read articles on how to connect different wallets that use seed phrases to my node but they require me to run an Electrum server which thing is waaay beyond my technical abilities as it has no GUI. Is there a "Connect your wallet to Core for Dummies" guide anywhere?
You can use Electrum without running your own server.
I love supporting the network by running Core
By default, Bitcoin Core isn't uploading data to other nodes.
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