when you say "I create all my backups on a file system level" do you mean you are backing up your entire linux? or the entire .bitcoin folder?
I don't care about the system files, usually I just backup my entire home directory. It does help I've symlinked my
blocks directory to another drive, so that's easily excluded.
and when you say "testing is as simple as running it in bitcoin core" are you saying just change out the existing wallet.dat file with the one in just copied?
Yes.
You can do that but only if Bitcoin Core isn't running or your backup could be corrupted (with low chance).
That is a slight risk indeed. I make sure I have enough backups anyway.
each wallets are inside a folder with a 'wallet name', then you don't have to rename them in the first place, the wallet name is that folder's name.
I'm old fashioned, still having multiple different .dat files in my .bitcoin directory.