Over-pumping can happen easily with an un-gauged pump, certainly. And definitely if you don't care oand want that blown-out look.
But there are pumps out there that have gauges on them such as mine:
(Not my ACTUAL pump but a pic of the identical style, with gauge)
As long as you're keeping an eye on that, you shouldn't have any issues with over-pumping.
I find the Bathmate (have the X30) a bit of a bother to use, the pumping action less than ideal. Having to push the cylinder into your crotch to get a bit of suction seems an awkward way to go, particularly compared to the hand pump seen here. I can easily get that same amount of negative pressure in just a few easy squeezes of the brass pump.
I'm also not sure how or why pumping out water instead of air is supposed to result in better... results.
That said, I've run into the same issue you did with my cylinder; I maxed out on the width already and I just got it a couple of weeks ago. OK, I basically maxed out right out of the gate. I ordered a tube 1/2" wider than me, as was recommended on several sites. Nope. I've now placed another order for a tube 1/2" wider still, so 1" wider in total, that should do me for a while. I may also order a 3" wide tube as I do like the feeling of having the entire package getting pumped, cock and balls.
But I'm too relatively new at all this to give much more than a cursory observation on the process and it's pros and cons between the two systems. So as I use both styles of pump, I may discover one
is better than the other in the area of results and understand why that might be.
But right now, the hand pump, using air vacuum, looks to be my preferred system.
And yes, everything I've read (that seems reliable and not just sales pitches) indicates that pumps are mainly good at girth, not length.