Did you consider that at 5200 feet the engine is receiving approximately 25 inches of manifold pressurem versus approximately 30 inches at Sea Level ? - That's a LARGE reduction in power, and accounts for the lower pitch attitude necessary at the higher altitude.
Going back to the original post (the reports of the aircraft "mushing: back onto the ground), if your rotation rate and attitude is the same as at sea level, with the MUCH reduced power, I'm not surprised at all. Gentler rotation to a lower pitch attitude is the name of the game
Regards,
Old Smokey