An issue has been created to implement support Wireguard-only peers like Mullvad exit nodes in Headscale:

https://github.com/juanfont/headscale/issues/1545

Headscale is a FOSS replacement for Tailscale's closed source coordination server. It is compatible with Tailscale's client apps, which are FOSS for Linux and Android, and partially closed source for macOS and Windows (https://tailscale.com/opensource/).