Have you considered having Headscale on a cheap VPS? We are actually doing that and it is pretty capable. IIRC, you can configure not to use the tailscale servers at all, and use your own public VPS for coordination. Bonus point, tailscale hired the Headscale developer and maintainer, and they are allowed to work on Headscale while on their payroll. The team looks very much into FOSS.