Most paid certs aren’t worth much anyway. Payment and delivery info for DV certs isn’t validated by anyone, it’s literally the same concept as Let’s Encrypt. OV and EV are the only ones that theoretically have any value, but nobody is using those ever since they got rid of the URL bar labeling; even Amazon is on DV nowadays.
Borg or the like with ‘hardcoded’ plaintext/regularly full-disk-encrypted key is acceptable. Someone that has your unencrypted private key sitting on your server has almost certainly already obtained access to the entire set of data you’re backing up, with the backup key itself only meaningfully guarding access to older backups.
The more important thing is to securely keep extra copies in case the server fails. I keep mine in a group in my password manager, one per repo.