Hi Rich - perhaps - there are a couple more things you can try, based on other users’ recent feedback on similar selection problems -
One user reports that, somewhat inexplicably, resetting all the checkboxes in the Display panel, that is, unchecking all, and then rechecking the ones you want, seems to have sorted it out. Not a solutuion but worth a try.
Try window selecting half of the file and hide or even delete - does anything change? Half again… and again, it might be that you can locate an object that causes the problem.
I think you said that in at least one file, you could hide some objects and the selection problem was less or gone, is that correct? Is that the file you sent me?