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!
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)