I had considered something similar, but recently I found that Ikea has started selling air purifiers and sensors for very reasonable money.
They even sell an activated carbon filter that you can add.
For years I had planned to make a DIY one, but never got round to it. But a week ago bought one of the IKEA ones, plus a carbon filter, and I've been very happy. The air feels cleaner and somehow just nicer.
Lots of reviews say it's underpowered for a normal sized room compared to alternative products. But for me, the alternative is a DIY one that I still wouldn't have got round to making.
I've been running one of these for a few months. The filter has slowly turned darker brown - which suggests it's working (not anywhere near forest fires here!).
It has a trickle mode which is completely silent and uses about 1W of power. Something you won't get on a DIY fan.
Could you confirm the technical spec of the filters you are using? Actual rating? Is it actually HEPA?
The ikea website seems to just be pretty vague marketing stuff with no details.
[0] https://www.wired.com/story/ikea-fornuftig-air-purifier/