Even by using Electrum, there are still ways that could screw your security that up. The reality is that you cannot really just trust anything.
When you use Electrum you don't have to trust anything, it is open source and you can verify the code, and it is also a self custodial wallet, so you control your keys and seed phrase, so most of your own part after verifying everything is then to ensure that an attacker does not get your keys or seed phrase.
It's what the community recommends that makes us trust it. I read what you said about not being connected in the internet at all, but you never know what you'll plug in into your device. I might sound like a real paranoid, but there's just no real 100% security.
There is no such thing as 100% security, but there are methods to achieve the best and most reliable kind of security, and one of that is by using well reviewed open source wallets, and by using them in a completely airgapped device. If your device is offline, you should never
plug anything into it, and never connect it to the internet.