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

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

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 familiar with your ISP’s TOS before doing that.