Must you give your mustard to everything?

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

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

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

Flame is not actively maintained anymore. Have a look at this fork which offers support for categories: https://github.com/fdarveau/flame :::

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/...

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.

This is the right answer.

Please get familiar with your ISP's TOS before doing that.