received debt letter from ROTC while on active duty, credit score took a major hit

After I left college, my detachment received a letter stating that I had outstanding debt, apparently had been overpaid book stipends (wasn’t on scholarship). Never noticed the extra pay, don’t think there was any, but I never saw the debt letter as I was gone and all my cadre were gone as well, replaced by new people. It was eventually forwarded to me months later from the new cadre, they told me to go to my new base and figure it out there and it wasn’t a big deal. Got to my new base, hit up finance, they said they’d work on it but weren’t sure what to do and that it would take some time. Left for training hoping they’d figure it out and it wouldn’t be a big deal, saw my credit score go down to about 450 from 800 ish in the meantime. Ended up just paying the debt of 2000 dollars as I didn’t want my credit to keep going down, finance said they couldn’t fix the credit score but it would go up slowly. This was almost two years ago, it’s been going up but very slowly- wondering if there’s anything I can do to fix the score, really don’t care about getting my money back, attempted to reach out to DFAS who initially wrote the letter about the debt but to no avail. Anyone been in a similar spot? Credit score is currently like 650, was hoping to take advantage of the military Amex deals (first word problems I know). Any help would be appreciated!