Hello Scott,
Sorry for the lateness of this reply, but hoping you at least got some off-line responses to your question.
My thoughts, solely as an amateur, but having kept my ears open over the years:
1) Just to be sure, you were confirming the Airthings results with independent devices/instrumentation? Many such devices certainly provide valid results, but all it takes is one "bad apple" to have you chasing after a phantom.
2) Curious if you have any idea what the ACH for the building is? Any sense of what airflows are like (volume rate and direction) through the envelope at different levels in the building?
3) For most buildings, I can't imagine that a bucket test on slab of only 7 pCi/L would be indicative of any serious infiltration or emanation, especially if your PFE is showing up to be good in the correct direction. But I am getting out of my depth here and would appreciate a real expert chiming in authoritatively.
4) The main suggestion I have is to do a series of samples that help you focus in on sources better. These would include:
a) Some IR imaging of surfaces that are soil-contact or of points that are possible pathway egresses. There may be some unusual point of entry that you've been missing, even with your PFE testing.
b) Some grab sampling for radon or progeny activity in a good variety of areas. I'm not suspecting the granite at all since the upstairs radon is around the typical ratio with the lower level's value.
c) Pay good attention to checking around those odd "extra-slab" areas such as entry pads, patios, garages. They can be (per Brodhead) their own high-radon micro-environments that can have a surprisingly outsized effect on internal levels.
d) Consider some gamma sampling if the above isn't helpful and you are still concerned about gross contamination, e.g., of building materials with Ra-226.
Good Luck!

Hi Scott. If possible could you post an update / if you found a solution? Thank you