Everything but the coordination server is open source. But you can selfhost this part yourself: https://headscale.net/

Tailscale is (like) a VPN, but traffic will go through their servers.

That’s wrong. In a peer to peer VPN the traffic should not pass Tailscale infrastructure.

See their docs: https://tailscale.com/kb/1094/is-all-traffic-routed-through-tailscale

Another very fast and folder based system would be mStream: https://mstream.io/

It works very well, also good mobile apps (Android + iOS)

It doesn’t matter how much you pay them. When you signed the contract you accepted their Terms of Service. Of course they can dictate what they want, you are free to go and choose another provider.

Please get familiär with your ISP’s TOS before doing that.