Can Bellamy have a spring in any part of him? How does he make his spring stiffer?

It's almost like Luffy's fruit, in terms of durability.

Can he make his spring stiffer?

Does it require lots of muscular strength for him to spring load his legs? Or can he fully control the springs from within them? I guess not. So he can't make them too stiff, cuz he won't be able to use it, unless he has a giant friend to compress him. This fruit is just pretty good, not great. It's just like normal muscles and tendons, but with springs instead of tendons.

How could Bellamy get stronger using his fruit? What could be his next "gear"?

Slick way to ask how he gets a boner.

If Oda has answered that Luffy's dick stretches and Buggy's detaches, you already know the answer to this.

Oda absolutely would answer yes to this in an SBS.