Looking like the root of my problem (other than slow internet at the school) is a Windows-only problem that's been around for 5+ years and is "on the issue tracker" at Claris and which basically comes down to "yeah Filemaker portals don't always work on Windows if you don't have a decent internet connection".
It's been a known issue with Claris in their issue tracker for many years.
There was a pre-Filemaker20 solution which involved a ReleaseDebugOn.txt (referenced in the link above) but that no longer works... but thanks to @WimDecorte and HOnza sharing knowledge on the Soliant Consulting website it looks like the solution is to use the same:
{“extraflags”: “no_portal_drs”}
Line in a ClarisConfig.json file:
It's a temporary fix which involves touching every computer to manually install this fix, but since this "temporary fix until Claris takes care of it" has worked with one change for 5 years, it's possibly more of a "temporary 1970's building" that's still in service now sort of temporary fix... 
Also, @steve_ssh thanks for reminding me of my 2020 post to the Claris forum about this same issue when all my teaching staff was trying to work remotely with their inconsistent 20Mbps down and 3Mbps up internet speeds, where Greg Lane, of Skeleton Key offers a simple script that will work great for the logging issues:
https://community.claris.com/en/s/question/0D53w000057cDtiCAE/built-in-speedtest-and-benchmarks