It's rare for entire houses to be affected, and the solution with ventilation often worsens by 50% in a couple of years. I think you should first focus on bedrooms and other places you spend a lot of time in. If there's high radiation in a simple partition wall, it's probably best to replace it. It's worse with exterior walls and tiled wet rooms, etc.
Instead of letting radon into the lungs and ventilating it, it should be stopped in the wall. You can put up a ventilated inner wall instead. Or a radon wallpaper where moisture shouldn't migrate in the basement, do they really work?
Should there be any sealing paint otherwise?
Perhaps it could be a good idea to add insulation from the outside and place a ventilated gap in between?
If high gamma radiation is a problem, it's just a matter of replacing it.
How common is it in beams as well?
Ground radon which is almost everywhere is a later issue and can become a suction. First, an honest gas measurement of the quantities, to compare with how the walls radiate.
