I seems a long time ago, I build the same thing as you. For templating I used github.com/christopher-ramirez/secretary and as “database” I used Google sheets. Still use it at work.
It never was really user friendly so I am looking to replace it. That is also the main issue with your self build solution? Or are there other issues as well?
I didn’t know about Documint. Looks interesting, will check it out, thanks 👍