An update a couple weeks later: The same issue seemed to persist for a long while. Just yesterday I decided to try a different stream type, and so far it looks stable.
Note that I am new to BI and its plethora of settings / options / etc and I don't really know why one choice here would be good or bad. But it seems like this is working better than the previous!

the change was from
Make: `Generic/ONVIF`
Model: `RTSP H.264/H.265/MPEG/MPEG4`
to
Make:`ReoLink`
Model: `H.264 RTSP`
I also happened to change the top option from `http://` paired with port `:443` to `https://` with no port, but that should mean the same thing.
It also looks like "decoder compatibility mode" switched from off -> on, but I don't remember doing that. YMMV hah.