I think I got it (somewhat) sorted.
I removed the ReaStream plugin, and added a send on the master track to “ReaRoute1/2”, and then in OBS added an ASIO input. Adding that input detected the Reaper ReaRoute, and my quick test had my DAW audio in the recording.
So, as far as I can tell, ReaStream still doesn’t work for me, and I don’t know why, but ReaRoute/ASIO should fill that gap. Fingers crossed…