Just chiming in. You can get damn fine gains with the 2" or 2.25. Sizing up, that will vary between men, it's good to think about future but enjoy the ride as well.
Just order accordingly, don't think that a bigger cylinder would automatically be better, especially in the beginning. (I confess, I think I made that error several years ago myself, as in purchasing a tube much bigger than needed for just cock pumping.)
The three issues you mention, yes, very solvable. The Mitysleeve itself can solve the suckage of testicles. And the sleeves transfer the force greatly! I had a session by Saturday using the longer sleeve and oh boy, haven't had such a session for a while!
All tools have a learning curve but you should be able to get a good seal easily with the Mityvac.
And lastly, yes, this is a big point, the pressure! Takes away the guessing game and you could be doing the low pressure, medium pressure and high pressure session so much easier.
(As in following the 5x5x3 routine, doing the first pump with lower pressure and increasing it rep by rep.)