I wanted to know if it's enough for me to simply backup the wallet.dat file on an encrypted USB stick?
Since bitcoin core doesn't support mnemonics that you could write down, the only option is to digitally store the wallet file itself. However, you may want to create more than one backup so that in case you lost the USB stick or it was damaged you could still recover your funds.
I also wonder, if this wallet.dat file is stolen by someone, but my wallet was encrypted, is the wallet.dat file encrypted? Or was that just a local encryption for my desktop.
The file itself is encrypted.
I would love to just be able to store my holy wallet.dat file away for many years and be able to re-gain access to my wallet using this file, primarily through other wallets too if Bitcoin Core wallet seizes to exist by then.
That should never happen considering that bitcoin core is the reference implementation of Bitcoin. But it is open source and many people are using it, so it would be trivial to write a simple script that decrypts and extracts the keys inside the wallet file to be used elsewhere.
And perhaps it's also possible, or necessary to store individual private keys in text on a seperate device such as an encrypted USB stick?
That's an overkill in my opinion.
I was thinking the private keys could be encoded in an encoding language to make them slightly extra secure incase any thief has no idea what he's doing.
A different encoding would provide no security if the thief knows he is stealing a bitcoin wallet.
Only a strong encryption can provide security.
Thanks a lot for your responses, very helpful.