There is a lot of discussion in the community about whether chastity will lead to penis shrinkage or not. Some would like it to, others not. As one who is locked-up long-term with just a few "out of the cage for a couple of days" events a year, I have no desire to have mine reduce, so I simply use a penis pump on the occasions when I'm out and give everything a good stretch. No idea if it helps but I can't believe that a bit of a workout like this does any harm at least. One thing that I'm well aware of happening with long-term chastity is reduced ejaculation strength on the three or four occasions a year this is allowed when the cage is off. This is presumably due to the muscles involved weakening with long periods of orgasm denial over a number of years.
Most of what I've read say any size reduction is temporary at most. I've never been locked for a really long time personally so I can't speak from experience.
Ocasional pumping is nothing permanent, it only draws more blood than normal and because of that it gets bigger, once the blood get back to the body nothing will change, suction if not regular will not do a miracle, if it makes you happy it is ok but i doubt it will have almost any impact on anything The muscles that pump the sperm are between the anus and the penis, you can work it even in chastity, every time you stop peing you are doing it with the same muscle, if you cum every day it will make more exercise and be in better shape, you only has to "stop" pee a few times a day and it will be the same as it used to be