I was joking before about needing wider rims, but maybe that IS your problem with the 2.6” tires. The main advantage of 2.6s is the lower pressure you can run on them without bottoming, but if you’re on skinny rims, you’re going to have sidewall flop unless you boost the pressure. When you boost the pressure though, you’re losing the smoother ride and better traction.
I also wonder if the tires are creating a weird contact patch by being pinched.
So basically, wider rims=lower pressure and all of its corresponding benefits with less flop. Eh?