Ok, I am still no doctor, but after three years of studying to become one I can tell you for sure that the doses of testosterone lost with semen by cis males are pretty insignificant. I have never heard one case of testosterone deficiency due to excessive masturbation, ahah!
Moreover for obvious reasons transmales do not produce semen, so cannot lose testosterone that way. If you think the changes are slow, it is probably due to genetic reasons. I mean, if you look around you'll surely see cis guys in their twenties who look like young boys, and still their bodies have been under the influence of testosterone for 10+ years. It's different for everyone. The only thing you can do is having your T levels regularly checked, and make sure they are in the male range. More than that you cannot do. Don't worry about masturbating too much.
About the article linked by Kreuzfidel, it's interesting, but I think it doesn't apply to transmen. As you said, it says that "excessive" masturbation can have effects on the brain and so reduce T production in the gonads. But again, in transmen T mostly comes from injections or whatever, so it shouldn't affect them.
