Hey, sorry if this is already too old, but I’ve talked with a friend about the similar issue you guys are experiencing. I put together a quick video on how I resolved the problem for myself with OBS. I hope it helps. Cheers!
And this is what it looks like in the end.
Oh, by the way, this is the original game window, and not the Steam VR mirror. I’ve also experienced lagging while trying it out with the Steam VR mirror.