Moved into new (to us, built in 1980) house 2 years ago. Installed an Airthings radon meter a year ago. Levels get up to 400 Bq/m3 when all the windows are closed. Finally had a radon mitigation system installed. Will hopefully get to much safer level below 100 Bq/m3.
Yes with basement as German houses mostly have. The values were worse there but still very bad at ground level.
Regarding cancer, it returned twice afaik. Non smoker or drinker, cycling in nature.