FWIW, most milb pitchers shut down sept, oct, nov and resume a throwing program december with bullpens beginning mid-late january, ML pitchers are on different schedules due to reporting to spring training. Your velocity should be around 75-80% when you arrive (not at full). ML pitcher have schedules depending on how deep they go past season. EVERYONE takes time off from pitching. Even position playres shut down for awhile.
But I agree with BOF, no reason why any young player should put in 150 innings in an adult league. A lot also depends on velo, young softer tossers can go longer innings per year than harder throwers. And dead arm can be a signal that something not good is happening. Some pitchers continue with dead arm, some miss a start with lighter pens.
I recommend that you contact your college pitching coach for training guidelines. Never compare yourself to a pro player, they get paid for more time on the mound and better trained.
I do not agree 100% what you feel about your velocity is normal, yes no one has their best stuff everyday, but that only means where you hit your spots, not if your velocity is up or down. And I agree it could be your mechanics. Check that out. And watch for a significant drop in velocity, which could signal a problem.