Essentially you have a sports car; not a boulevard cruiser. BUT, non-RFT tyres help a bit (Alpina Roadster S's don't have RFT's, and are a better ride than a std Z4 as a result; but are still relatively hard). Your next route to softening is smaller wheels and therefore bigger sidewalls....but that all depends what you have on there to start with. We have 17" winters to replace our 19" summers...that change is quite noticeable on ride quality. You can also source tyres from manufacturers who produce softer riding rubber by using online reviews as an indicator.
HTH