I would suggest an air monitoring tool like Airthings: www.airthings.com/wave-plus

This will monitor not just Radon but other contaminants.

Installed, dehumidifier, not one with a bucket you have to dump is probably a must. That or one that you can drain into the sump-pump.

Lastly a 20" boxfan filter. thenourishinggourmet.com/diy-air-filter-using-box… Its easy, cheap, and very effective.