After countless of trial and error.

I was not able to get MSFS 2020 Anniversary Edition Standard to run on steamdeck latest SteamOS 3.5.7

Tried the following launch option:

gamemoderun %command% PROTON_USE_DXVK=1 -FastLaunch

but still presented with unlimited loading black screen. Where the round spinner will spin for a while then hang till nobody business.

Tried multiple Proton Compatibility Runtime: (eg, Proton Experimental, Proton Latest, Proton GE-8) still was unable to get it running as well.

Also tried Proton Experimental - Bleeding Edge Beta, was out of luck as well.

From Proton DB, Seems like everyone able to launch with just Proton Experimental and “-FastLaunch” Launch option.

Not sure where i did wrong.