I wanted to know if it's enough for me to simply backup the wallet.dat file on an encrypted USB stick? This way I can practically remove Bitcoin Core from my desktop and if I ever wanted to send my funds elsewhere I can re-import the wallet.dat file again, but I can still send funds to this wallet.
That's a perfectly reasonable thing to do, and as others have alluded you should make multiple back ups, on different types of media such as CD and USB/MicorSD.
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.
If you need a password to spend from the wallet, then it is indeed encrypted. Even a wallet with exceptionally strong password/encryption that's been stolen should be considered compromised. Don't wait, move all funds immediately.
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? 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.
Recent releases of bitcoin core are hierarchical deterministic (HD) wallets, and you can use the HDSEED to backup and restore the wallet. An easy way to obtain your HD seed and back up several private keys (including imported ones) all at once is to use
dumpwallet. That will create a clear text file with a bunch of your private keys and associated addresses, but one key is of particular importance; it'll be labeled "hdseed1." It's a WIF private key, and it can be used to restore a bitcoin core HD wallet with all the other addresses and change addresses. It will not restore any private keys that were imported using
importprivkey or
importmulti.
Generally speaking I shy away from storing my backups digitally. I don't trust the archival longevity of digital media. The backups I trust most are seed phrases written on paper. A private key or a QR code on paper is also extremely robust, in my opinion. The most important thing, however is redundancy.