I'm using borg serve with the --append-only option set to have a server backup itself to a remote repository without being able to tamper with the backup data.

That's fine, but prune operations with the append-only mode activated of course doesn't free space. That's why I'm struggling how to handle this properly. I could run a local borg command which applies all transactions, but I would then have to trust there are no bad transactions already in the repository.

One may be aware of an attack after some time has already passed. That's why would like to have a borg command to "apply" all transactions within a certain period of time, e.g. older than 30 days, where I am (quite) sure the backup client hasn't been corrupted in this time.