Alright, let me give this a shot...
Only people on active duty IN THE NAVY are eligible for STA-21. So regardless of how long you were in any other branch, it doesn’t make you eligible.
We do have people in my unit here who are college programmers (not receiving NROTC scholarship but still do everything NROTC related in hopes of getting a scholarship) who are reservists. They still do drill once a month, but do NOT wear their Marine Corps uniforms to NROTC events. For our purposes here, even though they are still Marines, for Battalion events, they are Midshipmen and wear Midshipmen uniforms, follow Midn regs, and are subject to Midn chain of command just like all the other 17 and 18 and 19 year olds who have zero prior experience.
People always say that prior service in any branch will help you. I always say that it can make you or break you depending on how you conduct yourself. If you are a prior service Army who just marked time for four years until you could get out so you could “start over” in the Navy/Marine Corps, then that will come out and your prior service will be held against you, i.e. “I know him, he was in the Army before and man, was he a turd.” Because Army, Naval, and Air Force ROTC units at the same schools see plenty of each other and you get to know people from the other services. However, if you were a stellar performer in the Army and tried to change over, hey, it could work for you.
My question for you. Why the Army? You sound pretty interested in NROTC, so why even test drive the Army? Not saying you shouldn’t, but I knew I wanted to be a Marine officer when I was in high school. So when I didn’t get the scholarship, I didn’t talk to the Army or Air Force recruiter, I enlisted in the Marine Corps (active duty). Just something to think about.
Whatever you do, think long and hard and don’t make any rash decisions. Good luck!