I’m having second thoughts about migrating towards borg backup

The main issue is how to handle “3rd strand” backup. I use a removable HDD that I sync with my main backups monthly and lives outside my house the rest of time

Borg strongly discourages syncing their backup repos using file-level tools like rsync.

AFAICT (we borg quite a bit) this is because they make no guarantees as to how much of the repo they touch during a backup, and rsyncing during such an event might result in a broken 3rd strand. So long as you're careful to not hit that, you should be fine.

Yeah, I’ll arrange all the jobs to be mutex (via systemd units). It still doesn’t give me the warm fuzzies. There’s some potential issue about a client (eg a restore operation) being unhappy if you ask it to talk to two repos with the same uuid