for the record; I think I’ve managed to clean out all the extraneous dispatch stuff and priorities now and it seems to work as expected (testing in progress though…), have to be careful about the initial suspension point though, but that is a different issue.
Funnily enough, due to other optimizations the processing now is so fast that filtering 10+ million items on multiple properties is less than the debounce time threshold for displaying a progress bar (300ms) so one of the original drivers looking at this disappeared. Thanks for the replies that helped push me in the proper direction.