The tailscale daemon/CLI/client code is already open source and works with the above as the control server.
The tailscale team appear to be encouraging development of headscale too:
"Our opinion is that Headscale provides a valuable complement to Tailscale: It helps personal users better understand both how Tailscale works and how to run a coordination server at home. As such, Tailscale works with Headscale maintainers when making changes to Tailscale clients that might affect how the Headscale coordination server works, to ensure ongoing compatibility."
> https://tailscale.com/opensource/
Personally I find WireGuard and tailscale/headscale to be extremely complementary, and with these you don't cede any control vs running WireGuard on its own.