No sure why you are trying to bring up the overlay. You just need to enable it in GEFORCE EXPERIENCE app from Nvidia (if you can bring up ALT Z then is is enabled). You stated you checked in the steam properties for the MSFS2020 game in your Steam library and it is enabled there so not sure why it is not working for you.
When you buy and download a popup from Steam should come up and allow you to purchase that market content through steam.
Try to change MSFS2020 in game control graphic setting to WINDOWED if you currently have it set to full screen and exit the game and back in and try the marketplace again. Also I would make sure the Steam icon is showing in your windows 10 system tray and you didn’t close it earlier (although you’d think it would be running in the background if MSFS2020 was running).
Other than that the LINK i provided on my previous post is from the Zendesk help.