The issue of doing in-house is the issue of finding competent devs, meaning not only "someone who code" but someone who understand how to PROPER design things, witch is essentially a lost art.

EDIFACT is dramatically simple, far more than modern XML-based peppol alike dialects, here the issue is "just" knowing that very likely any "institution" might have injected a gazillion of personal stuff, mostly undocumented or badly documented to be take into account for a long interim, but developing a personal ERP trying the modern path is far from being digestible.

IMVHO it's feasible trying to fish common-lisp/clojure world. That's a niche enough there is not much retention issue, and there are still skilled devs because it's a kind of niche where some arrive, try, fail and go, some skillful remain and do want to be there.