It sounds like you're doing it right. My old basement could be sealed with a lot of work. So it's not gonna happen.
I looked at the afternoon 24 hour plot and saw a 4.6 peak at 17:30. The oil burner ran from 17:00 until 17:20. (We are trying to make this tank last until WW3 starts)
It's clearly sucking up some radon and shooting some up the chimney. I'll bet most of the houses on this granite hill do the same thing.
The good news is, two of the new families on my street have installed Mini-Splits during the last month.
The new kids next door went with Propane heat. They are already talking about moving to Florida (from MA).
I don't blame them.