I tried to use Little Navmap today. And guess what? It works! Plan a flight, connecting to fs2020 and importing the flight. I tried it with the BTM and AP nav mode followed my flight plan very nicely!
Might need FSX SP2 SimConnect , not sure: https://www.littlenavmap.org/downloads/SimConnect/
Connect for live following your and AI planes “Connect directly to local X-plane simulation”
To import a .pln flightplan just press “Space” in the world map of fs2020.
Check this discusion: https://www.avsim.com/forums/topic/579108-lnm-and-microsoft-fs2020/page/2/