The short answer is get a Mullet. I’m 6’ 3", 200 lbs and have size 14 feet. I took a Braaap down some class IV and it was fun but I was hilariously heavy/big for it although it was actually surprisingly comfortable. I’ve never been in a Mullet but I understand it’s like a big Braaap. You could also look at the Jackson Antix or Dagger Axiom.
Getting a Mullet-style boat is a great way to make whitewater you find easy more challenging. Probably not a bad idea to get one. I’d hesitate to sell your Mamba though. Some rivers aren’t suited to a slicey boat (Braaap down Paully was rather miserable since the stern kept slicing deep and hitting the bottom).