So I chose to update this old Bitcoin Core wallet.dat step-by-step with each intermediate Bitcoin Core version from 2014 to 2021 and this worked for me in this particular case. There were IIRC about two Bitcoin Core versions which required changes and updates in the wallet.dat "structures", I guess probably those were the versions I shouldn't have skipped over.
Last used in 2014 implies that you were using Bitcoin version 0.8 or 0.9 or something earlier.
I don't think Bitcoin Core has a backup keys feature for this purpose (dumpwallet is probably the closest we will get to a backup/restore medium).