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.
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That is indeed possible. However, depending on your actual requirements, may not be the best strategy.
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 data stored in the wallet.dat 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.
Note that while the actual format of the wallet.dat is published and "known"... it is a format that is pretty much exclusive to Bitcoin Core (and some of it's spinoffs like Bitcoin Knots). If you're concerned about the long term, you might want to consider other backup options (or perhaps other wallets).
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.
Personally, I wouldn't go trying to create "clever" encryption systems... That often creates more issues than it solves.