Very far from an expert here, but just Fermi estimating some numbers:
This cheap lamp on amazon puts out 25 watts of 254nm light. This paper says 222nm light is similarly effective and takes around 2mJ/cm2 to effectively inhibit viral replication. So if you were to put this light somewhere where it was shining on, say, 3 square feet of ventilation duct surface, and assuming 50% efficiency at the power translating into light hitting that surface, we'd have 25*0.5/(12*2.54)2 = 13 mW/cm2, which seems like it leaves room for substantial airflow (multiple air changes through the lighted region per second) while remaining effective against viruses.
(Cost of running this lamp 24/7 would be 8760 hours * 0.025kW * $0.14/kwh in the US = $30 per year. More if you cover multiple ducts with multiple lamps.)
For mold spores, this paper (sci-hub link) seems relevant? I'm not sure I'm following their protocol exactly - they indicate some amount of UV inactivation of fungal spores that seems to get good results in a room within 2 hours, but it seems like maybe this is measuring from UV light shining directly onto an air filter's collection device rather than over air that quickly cycles through? Though if these air filters (some of which have internal UV) are capturing the mold spores and then deactivating them, maybe you can just use those. The paper looks at ozone concentration and finds that it stabilizes at less than, but not vastly less than, the FDA limit of 50ppb (I have no idea how strict that limit is). Seems substantially less effective than it is against viruses though, which is what I'd expect given that the mold spores are much larger.
Just get a UVC bulb and use whatever fixture you might have. https://www.amazon.com/dp/B08HFKMD2S
We went this route https://www.amazon.com/dp/B0989QGCDB
and connected to a smart switch (or "manual" switch) and leave it on for 10min at a time. You have to be cautious using it, so it would be wise to read up on proper use of it.
Any LED based "UV" "Sanitizing" light are fakes.