I sympathize. The problem became almost unnoticeable when I turned the detail settings in the game down to as low as they would go.
We got as far as determining that the problem is probably somewhere between the interface between OpenXR and the WinMR stack, discovered that both SteamVR and lowering detail got it working, and dropped the issue, because the number of those headsets still in use are in a relatively easily countable range. I know you only had a year to prepare for that, but it’s been broken in Unreal for a lot longer than a year.
I don’t know if 5.0 or 5.1 work with WMR reasonably accurately, but 4.27 does. You might also just be able to install SteamVR on the PCs, set the XR stack to SteamVR, and use the Unreal SteamVR plugin.