Derailleur shifting into spokes and breaking frame is not a frame defect and not Cannondale's problem. Misadjustment, wear, damage to a derailleur on a bike that is eight years old is not Cannondale's problem. Misadjustment might be the bike shop's problem if they adjusted the derailleur immediately before the incident, but as weeks or months pass, it stops being their problem, and wear or damage was never their problem anyway. Finally, by removing the spoke protector and riding the bike without it, the OP invited a derailleur-into-spokes incident. I remove the dork disc from my bikes, but I leave them on other people's bikes.

I don't think Cannondale owes the OP anything.

Furthermore, 20% of a new Cannondale frame is about equivalent to the value of an eight year old Cannondale frame. So that offer is generous and OP should stop whining and accept it.

And don't call it a "$3000 bike". It is a $500 bike, maybe (check Craigslist prices for eight year old Synapses) including a $250 frame, at best.