Keeping the butt strap down should be accomplished by the leg straps. Try hitching the belt lower on your hips, then tightening the leg straps more. I wear my belt considerably lower on my hips than I would a belt to hold up my pants. Since most belts don't have any adjustment of the distance from the butt strap to the belt, you will get more butt onto your butt strap if you lower the belt.
Some guys won't tighten the leg straps much, 'cause they think they are saving the family jewels. If you shift wrong in the saddle, those legs straps are more likely to hurt your privates if the butt strap slides up your back than if it stayed held down low where it belongs. Riding your crotch onto the legs straps is not a fun way to climb, so make sure they are tight enough to do the job right. Tighter isn't necessarily tighter where it hurts, see?
Since you bend at the waist (not the hips), you should also have more flexibility and freedom of movement if you lower the belt. If you are weak in the waist, this will make it more difficult to reach out horizontally, since you will have more upper body hanging out unsupported.
I don't like a floating bridge. I like being able to push sideways against the rope, and a floating bridge feels like I have nothing to push against. A lighter, more athletic climber than myself might feel like they have more freedom of movement with a floating bridge.
There are probably as many threads on saddle choices as there are on "which 2-cycle oil" or "which friction hitch", so use your search feature to find hundreds of opinions on any given saddle.