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Possibly not enough HSC - those shocks (DPX2, SDLX, Float X) often come with tunes that have very soft high speed which would feel how you describe. They just blow through travel too easily and don't control the impacts well enough. The Float X2 usually has a lot more damping than all of those shocks as well as a more linear spring curve. The Float X is probably easiest to retune with a more appropriate setting, especially if you remove the extra free bleed from the base valve. It has a fairly large volume air spring too so I find...

My guess would be that with the smaller volume shocks you are running into the progression ramp of both the shock and the rear suspension design. You're hitting the progression ramp and your ankles can't support the force anymore and are straining. That's why the X2 feels better but uses less travel. With the smaller shocks you're blowing through travel without dissipating the energy and crashing into the ramp up. I would try removing volume reducers but increasing compression especially low speed. More LSC should help prevent blowing through travel from body movement. You may also need to increase pressure...