@christianp I run pixie.town and for it's backups I use Borg (with Borgmatic on some servers, and there's also Vorta which has a nice gui). Storage backend-wise I've used https://borgbase.com, BuyVM with a KVM storage slab, and a Hetzner storage box (definitely slower, but cheaper too).
Remote machines also back up to my local servers, but that's rather dependent on your network speed and if you pay for traffic. A Pi with an external disk at someone else's home could be an option too, the newer pi's have not-awful disk throughput i think