You know that TOTP is actually one fairly simple algorithm? You can write your own.
Or you can have all of your Key URIs in a plain text file encrypted with let’s say GPG then use them with something like oathtool.
I have the base32 secrets for each of my TOTPs in several places, not because of backup purposes but because I have a messy setup.
At the very end you can have a plain file simply zipped with encryption synced in any cloud service of your liking.