I can vouch for the node 804, although I haven’t used the others so I can’t say which is the best.
I’ve been using Kopia for all my backups for a couple years, both backing up my desktop and containers. It’s been very reliable, and it has nice features like being able to mount a backup.
You couldn’t make a proprietary server. Client is fine, AGPL doesn’t apply when you are accessing the server over a public API.