> Yes, a backup tool never deletes by itself, but the standard way of using such tools is that you keep the last n snapshots - this is just delayed sync.

Restic, Kopia and most other modern backup solutions allow you to to define the retention policy. Usually by specifying how many hourly/daily/weekly/monthly/yearly snapshots to keep. They don't just remove all snapshots that are older than n.

They also usually let you restore any given file, from any given snapshot, without having to restore everything. And as long as there's a functional index, this shouldn't be terribly slow either.