I’m on the free plan at the moment, and it’s pretty neat, but I’d actually be willing to pay for a self-hosted version :P (But I guess the existence of such an option might tempt some paying enterprise customers to attempt self-hosting instead?)

(I’m aware of headscale as an open-source control plane, but the iOS client is still closed-source and hard-coded to only use the first-party control plane :( )

While not apples-to-apples and less polished, we're slowly building up https://github.com/tonarino/innernet as a fully open-source (and self-hosted) alternative to things like Tailscale. It controls vanilla WireGuard under the hood (kernel or userspace implementations), and is lower level (no graphical interfaces yet), though, but depending on your needs it might still fit :).