Bass saxophone is perhaps the hardest of all saxophones to master. It doesn't matter what type bass you play - they are all difficult.
Remember that everything is bigger. Take a bigger bite (bigger is only a small amount of mouthpiece, but it's a BIG mouthpiece, so you'll have to open your mouth. Take a bigger breath. Part of the mechanism that drives a bass sax is inside your body. A larger resonating chamber BEFORE the horn starts is an asset. It's all in the Benade book, but it's complicated. JUST TAKE A BIGGER BREATH. It solves so many problems.
Wiggling your fingers is not a problem, even on an old mechanism. Old Conns and Bueschers are among the easiest bass saxes to play. If something doesn't work, especially across the C#-D break, take bigger breath. Problem solved. Anybody can play fast. A great sound takes forever.