currently I can only set the prune as the number of days to keep. it would be useful to be able to set it up with more detailed policies, for example with GFS (last 7 daily backups, last 4 weekly backups, last 12 monthly backups).
GFS is one type of backup policy: https://en.m.wikipedia.org/wiki/Backup_rotation_scheme
Grandfather-father-son backup is a common rotation scheme for backup media in which there are three or more backup cycles, such as daily, weekly and monthly. The daily backups are rotated on a daily basis using a FIFO system as above. The weekly backups are similarly rotated on a weekly basis, and the monthly backup on a monthly basis. In addition, quarterly, half-yearly, and/or annual backups could also be separately retained. Often some of these backups are removed from the site for safekeeping and disaster recovery purposes.