zfs send

Proxmox is pretty much focused on ZFS, LXC containers and VMs. You want mergerFS and Docker. I say avoid Proxmox and go for Debian or another distro.

Only when using zfs, which op is not.

I always wondered the same about ZeroTier. If the controller is under foreign control, isn’t the whole network compromised?

There are often some “fair use” paragraphs in their respective ToS that they could enforce and either terminate your account or request you to uprade to a higher tier product. Usually (not always) VPSs are overprovisioned, so when people start to fully utilize their rented machines theit whole business model goes belly-up.

Be careful with compute intensive tasks. Some providers don’t like when you actually utilize your rented hosts.

Not exactly federated, but open source: https://github.com/Alovoa/alovoa

What you call an algorithm here is a recommendation engine. I don’t see why they should avoid having something like that. Ideally they would have a modular system, so you could plug in your own favorite third-party engine.