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!
I’d like to offer a reframe that’s been helpful to me, if I may.
The inconsistency will haunt you throughout your entire life, but only if you let it. Learning how to embrace and understand my emotions (instead of controlling [or suppressing] them) has been incredibly worthwhile to me. Therapy is a good avenue for this, but I understand it’s both expensive and not yet fully accepted by American culture. I also recommend the book The Tao of Fully Feeling, and here’s an excerpt: https://www.pete-walker.com/taoFullyFeeling.htm