The problem was definitely fixed by re-installing windows, which was suggested early on, thanks @Djibouti62 . Did not discount it at the time, but felt other possible fixes needed to be explored first.
Having explored them all, finally came back to re-install windows and 2d 4k went from low 30s to low 70s.
But many thanks also and especially to @kido007dz @xxYUNIORxx and everyone else who helped guide me through all those things to try.
For VR testing I started out at the default Varjo 35 ppd resolution, which I always used on my old machine
and it did quite well, was very smooth
then I switched back to 39ppd
it dropped into the 30s but it was a pretty smooth 30 fps, not as smooth as at 35ppd, but the visuals are so much better I want to stay there, and it is good enough. Will see what a little tweaking can do but really this is the best VR experience for me so far, visuals much better than my old 3090 5950x machine, and smoothness is better too.