I use Nebula because its iOS client does not drain my battery. Tailscale has had that known bug for years and they never managed to fix it, which is a major deal breaker.

They have released a slew of updates recently to fix this, and they did a complete rewrite of the Android app

OpenZiti would be another - https://openziti.io/. I work on the project. 1 issue with Nebula is the provisioning new clients with identities. Its not completely open sourced by the Nebula company.