Confused by your post since MSFS will use OpenXR and nothing else. Here we’re talking about SteamVR OpenXR vs OpenXR for WMR.
If you cannot use Motion Smoothing in SteamVR OpenXR, perhaps try the following:
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Run
C:\Program Files (x86)\Steam\steamapps\common\MixedRealityVRDriver\bin\win64\OpenVRSettingsUX.exe -
Under “Graphics” check your current settings:
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If you have “Disabled” then it means you will never get Motion Smoothing, even when enabled in SteamVR. You will need to switch to “SteamVR per-app setting” or even try “Motion Vector” for force enabling it.
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Under “Developer”, you can also enable the status indicator:
This will show a square on screen when Motion Smoothing is disabled. You can look at Using SteamVR with Windows Mixed Reality - Enthusiast Guide | Microsoft Learn under “Motion Reprojection Indicator” to see the different meanings.