Recall some time back reading a non-authoritative article to the effect that (as with many houses 'infected' with degrees of, debilitating to humans -type spores) .. many of us are alleged to be carrying around long-running fungi in & around sinus.
I haven't investigated further, but Prednisone was suggested in article as the likely nostrum: yet I can recall that even decades ago, thet was the 2nd pill for a new MD to prescribe when they hadn't the fuck of an idea what might really be the problem. Bet - the cost for analysis means any HMO will NOT spend the $ to find out; WILL grab some horrific steroid and send you on your way.
(Dunno what fungal tests would be like; since I have a kinda chronic sniff - but not nearly at the level you describe + only occasional sneezes: keep us informed if you get a diagnosis more believable than a Microsoft Fix-pack, OK?)
It's on my list (to find out) - know a local osteopath with a great track record. There are so few *wise* practitioners of Anything, left - we're all in the same boat: living in a society which runs on dissembling & hype and phony machine-produced 'Profe$$ionals' ie Capitalist Corp Medicine. :(
Ashton