Storing encryption keys for backup drives

I'm planning to setup backup on my nas with the 3-2-1 backup rule.

For the backup disks I want full disk encryption, but I also want to be really sure that I don't lose the encryption keys if I lose my phone and computer where I have my password manager.

What is a good practice to store the encryption key(s)?

One thought I had was to have an unencrypted partition on the backup disks that stores an encrypted keepass database with the key.

Any tips or experiences are welcome.

PS. I want to avoid cloud-based options.