Allergies, cold, irritants??? need an Air Purifier? not sure.. HELP?

Been living in the same house for 8 years. Old home was redone before we bought it. When we moved in, we fixed a leaky roof in the back that had caused a little bit of mold on the ceiling. All fixed, no issues. Still have some problems with mold growth in the bathroom, however. Vent fan must not be cutting it or something.. I dunno. But I do my best to keep ontop of cleaning and sterilizing until we have saved up enough to completely gut and redo the bathroom.
Anyways, since moving in, we have had 2 kids.
I have never had issues with allergies until around the time I had moved here. (Could be from farmland though, as I had moved from the city.)

The point is, my husband gets reoccuring sinus and upper-resperatory infections. Our daughters get them every year as well, and I have seasonal allergies (or so i presume) but it seems all our symptoms get worse when it’s damp outside (after a rain, or when snow starts melting). Could this be a mold allergy? Symptoms also get worse during the night.

We put in a new furnace a couple of months ago and so i now know it’s not from dust (we had all ductwork thoroughly cleaned as well).

Thinking an air purifier may be our last hope. My MIL bought an edenpure ozone generating one. They have no problems with allergies at their home, and it seems mine get better for a while when i visit. I don’t think an ozone one would be a good idea with kids in the home though, so I am looking at a Rabbit Air BioGS 421A .

Does anyone have a clue as to what this all could be? Would an air purifier really make a difference?
My allergy symptoms also were almost gone visiting his grandparents over Christmas. They have a very clean home AND a cheaper honeywell air purifier…. I’m just wondering why symptoms seem worse at home? I dunno… what is mold allergy like?

Can anyone give me any real advice or help? Thanks!