For PM2.5, the Qingping Air Monitor (or the Lite version if you prefer that) is good. The review I've read found it has decent accuracy, and, though I haven't tested it rigorously myself, the readings seem reasonable in my experience. The main reason I recommend it, though, is that the UI is great: it keeps graphs of past values that are easy to access, both on the screen and in the app.

On the topic of allergens, I wasn't able to find any device that can measure that.