"Flying sailplanes as a Sport Pilot would limit you to flights less than
10,000 feet MSL (major problem where I fly) and a sailplane that has a Vne
of less than 120 kts."

Not quite true. There is an exception to flying over 10,000 feet. I recall
it is a maximum clearance. Also, the maximum speed is based upon level
flight operation and is not Vne. It is Vh.

Colin