Quick update - have had a good clean up and reinstall of drivers etc (incl AMD drivers for CPU). I have narrowed the issue down to the Steam version of Forza Horizon 5. I cannot get much about 90FPS on that version but running the native windows version i get around 140fps (both exactly the same graphics settings). I can also now get around 140fps in MSFS using frame generation.
Going to try a bit of overclocking of CPU next to see if that makes a further difference.
Thanks for everyone taking time to provide help - much appreciated
If you had tested more than one game you'd have found this out much sooner; that's why we should not draw conclusions from a single source of data