Thanks for all the work you put into this, much appreciated!
I did notice that a flight plan created in Little Navmap and imported/loaded into the sim (.pln file) only shows the departure/destination in the Legs screen on the FMC, no waypoints.
However flight plans created in MSFS2020 will show complete routing in the FMC.
I read about the setting in the FMS to Sync with ATC, but even when selected, it always remains OFF (green).