1. So, you do want to run rsnapshot on the Borg repository (the destination to which is backed up)? Both rsnapshot and Borg keep a history, so you are keeping a history of when the Borg repository had which history. This will not be particularly efficient nor “as intended”.
2. be aware that Borg does incremental backups on file chunks, while rsnapshot works on whole files. So if a large file changes, rsnapshot will duplicate the storage used.
3. a Borg repository is more like a database of chunks (similar to git), while rsnapshot recreates the original backup data.

According to borgbackup.readthedocs.io/…/data-structures.html the repository storage consists of 500mb files. So using rsnapshot on those, will not be work as you want it to.