1. Back up Core.
You should have done this already (the file
wallet.dat is the only one you need to backup)
2. Set Up Electrum.
3. Back up Electrum.
4. Import private key to Electrum.
5. Back Up Electrum.
Electrum has different wallet types: the "Standard wallet" will get you 12 seed words to write down (that's your backup), but can't import. The "Import Bitcoin addresses or private keys" wallet allows you to import, after which you can make a backup. Or just keep your original wallet.dat as backup for those private keys.
Or, don't use "import private keys", and only create a "Standard wallet". Then, use: Wallet > Private keys > Sweep. This moves the funds from your old address directly to your new Electrum wallet.
6. Set Up any simple and convenient moblie wallet like Bitcoin Wallet.
Bitcoin Wallet is OLD! I've used it, but it takes
forever to update an old wallet. Did you know there's also Electrum for mobile?
Use it to spend btc the following way: receive small amounts from Electrum and immediately send them further.
If you immediately send them again, what's the point of using the mobile wallet? You can just as well send it from Electrum directly.
My system is to keep a small Bitcoin balance on my mobile wallet all the time, which is what I use that for daily expenses.
Thus, Electrum is online always just for a short time.
Malware only needs a short time. Once your wallet has touched an online computer, it doesn't help anymore if you take it offline. If it's compromised, your funds will be taken.
7. Buy a hardware wallet.
Set it up.
Transfer btc from Electrum into that hardware wallet.
Hold btc on hardware wallet.
Go!
8. Electrum has zero btc balance after btc have arrived to hardware wallet, and thus Electrum became good to claim forks.
Electrum itself can't claim forks, but other than that, once your Bitcoins are safe, you can claim them. Note that the values in my guide are outdated. You're looking at about $200 worth of Forkcoins (in total) for each Bitcoin you owned when they forked. That's only 1% of the value of your Bitcoin, so don't rush it.
After I looked on all those wallets at bitcoin.org I started thinking that probably hardware wallet is best choice. But as it takes time to order and get it, I need to choose some desktop wallet now.
For what it's worth: You've waited since 2015. Don't rush things now