I add such stuff to my password manager. It supports files. But not all password managers do. I have a category for admin stuff where I also save passwords to servers, database credentials, service logins and the exported LUKS keys of the harddrives. I'd add backup keys there, too, but I currently keep them unencrypted on an encrypted harddisk.

For anything that is related to my backup scheme, it's printed out hard copy, put in an envelope in a fire safe in my house. I can tell you from experience there is nothing more stressful than "oh fuck I need my backups but the key to unlock the backups is in the backups fuck fuck fuck".

And for future reference, anyone thinking about breaking into my house to get access to my backups just DM me, I'm sure we can come to an arrangement that's less hassle for both of us

Password manager. Many allow you to attach a file to a set of credentials

Bitwarden let's you upload files (key files) and save all you passwords.

There are many ways to go about this. Files like those keyfiles and encryption headers are extra sensitive because (a) they potentially provide access to everything and (b) losing them can block access to everything. Personally, I keep those types of files unencrypted in a directory that stays 100% offline (encrypted backups to external disks only). But there's no reason not to back those files up to an encrypted online repository (where you trust the encryption). Just make sure that's not your only backup of those files for obvious reasons.

A good practice to avoid painting yourself in a corner is to test your backups: Switch off your PC / server, put your mobile devices in a drawer (pretend they're gone), borrow / wipe a cheap laptop. How do you access your backup files using just that laptop?

I just encrypt the whole backup drive and sync the files to that drive.

Yes, you should have backups. You can use something like KeePass to store them I suppose. I personally just use the file system on a secure server.