I don't know if Tailscale counts because it's mostly open source (with options to run your own server), but I use it constantly to connect to Home Assistant and Jellyfin on my home server, as well as pairing it with NextDNS (pihole is possible for those that want to go that route) for ad blocking and Mullvad to use them as an exit node.
You can selfhost it with headscale (the server). It's really simple to set up and use.
I'm also considering moving to zerotier because a) it's completely opensource and b) the wifi management software I'm looking into (openwisp) has native integration
I haven't used tailscale to know how well it works but as a current zerotier user I've been considering moving away from it.
I actually love the idea and it's super simple to set up but has some very annoying pitfalls for me:
It's a lot of "magic". When it fails to work the zerotier software gives you very little information on why.
The NAT tunneling can be iffy. I had it fail to work in some public WiFis, occasionally failed to work on mobile internet (same phone and network when it otherwise works). Restarting the app, reconnecting and so on can often help but it's not super reliable IMO.
Just recently I've had to uninstall the app restart my Mac, reinstall the app to get it to work again - there were no changes that made it stop, it just decided it's had enough one day to the next and as in point 1, it doesn't tell you much over whether it's connected or not.
Pretty much all of the issues I've had were with devices that have to disconnect and re-connect from the network and/or devices that move between different networks (like laptop, phone). On my router, it's been super stable. Point is, your mileage may vary - it's worth trying but there are definitely issues.
good to know, thank you for the insights! Tbh Tailscale/headscale has been quite stable, so maybe I'll stay were I am. Or move to nebula because why not? :D