I agree that straight up using Tailscale would likely be easier. But to answer your question, you’re looking to “push routes” because what you actually want to do is “route” but that’s kinda hard to Google haha. This looks maybe promising: https://forums.freebsd.org/threads/wireguard-how-to-route-another-subnet-through-it.89744/

This approach largely works, with the caveat that it then requires you to always be on the tailnet. If someone wants to connect locally AND via tailnet using the same URL, they’ll need to push/advertise routes (or do some other hacky thing)

Right now, I’ve only got the spoons to provide rough guidance, not details. In order to use non-tailnet IPs, you’ll need to configure your tailnet host to “advertise routes/push routes”. In more laymen terms, tailnet needs to say, “hey network client, I do know where 192.168.0.69 is! So I can route that request”. By default, each tailnet host only advertises the other tailnet hosts. Anything else fails.

Also, I really appreciate how detailed your question is!

To add, here’s an example of my OpenVPN config addition to ensure 192.168.3.* is accessible over VPN

verb 5
push "route 192.168.3.0 255.255.255.0 vpn_gateway"

Awesome! Thanks for the detailed update, and I’m glad it worked well for you!