Hey... does anyone have any thoughts on whether there's any downside to exiting FS2020 by using <alt>+F4 or closing the window vs. the "right" way?  At the end of a flight the process of exiting via the main menu takes several minutes and it seems like a waste of time if you're just going to exit the program.  But I'm worried exiting the fast way could break something...

My only thought and I have no real data for this is when you properly quit a program it commits any changes to the drive etc. If you force quit it using Alt-F4 you kill the process and changes to the files can potentially be lost.  What this does for your overall sim is anyones guess.  It can be lost setting configurations, corrupted files etc.  I wouldn't recommend it as a regular process personally.

Maybe you don't get an (correct) entry in the flight log?

I run in full screen when using the program and originally I would quit back to the main menu before requesting it to close the program from there. Since then i've been pressing Alt & Enter when i'm finished to activated windowed mode and click the red cross in the top corner, it's just so much quicker. I haven't noticed any difference in behaviour at all.