Pruning takes time so needs to be done as less as possible.
AISbackup already estimates if the backup will fit.
If it fits then simply start the backup.
If it doesn't fit calculate the amount of space is freed up by:
a] Prune the zips.
b] Deleting that 1 deleted copy that's in the zips. (if configured to keep it)
Based on the calculations take action. That can be automatically if configured by the user, or give the user an option what to do.
a] "Pruning will free up enough space. Prune?"
b] "Deleting deleted files will free up enough space. Delete?"
c] "Doing both will free up enough space. Proceed?"
d] "The backup won't fit. Do c and try anyway?"