Tailscale is an automatic no for me. I won’t use proprietary software for something as important as this. Even with tincs history I’d much rather go with opensource.
If you haven't seen it, an open source implementation of the server: https://github.com/juanfont/headscale.
The Linux & Android clients are fully open source. The Mac GUI is closed, but the "Linux" client runs on macOS, sans GUI. (Also runs on BSDs, and I believe Windows)
At that point, only the iOS and Windows GUI app is closed, but they're both super thin wrappers around the open source Go implementations.
But, yes, it's true that we're not entirely open. We'd planned to release a simple server implementation when we got it cleaned up, but then Headscale beat us. That seems like the most important missing bit.