I would love to learn more about how to set something like this up. I hope this group will consider open-sourcing more tools and instructions for others who hope to follow in their footsteps.

As some other commenters have said, Headscale/Tailscale scratches some of this itch for me. Having individual devices connect directly to an overlay network is fabulous. However, I can never fully control Tailscale the way I wish to, and connecting networks together with it is difficult.