Shame to hear that you guys are having suspension issues. This above contrasts directly with my own experience. Mine is a 320i T MSPort on 19" Goodyear RFTs (coincidentally also bought new as better deal than used).
I find the ride comfortable. Sure, its no magic carpet, but then the 3 series is not that kind of setup so would be defeatist to expect it. Certainly not "back-breaking", more like taut and responsive. Its as smooth as my old 530i E39 SE Touring and miles better than our 325i E91 SE Touring. Plus, it handles really well.
When I was testing G21s I found the ride and noise on 18" wheels much better than 19". Would have preferred that setup but my car came as a pre-configured deal so had to take the (lovely looking) 19" wheels.
Falcoray I think it would be crazy expensive to replace the whole suspension, not to mention you'd end up with a "Frankenstein car". What would you replace it with, the setup from an SE version?
You can find all the part numbers on RealOEM:
https://www.realoem.com/