[codingdave]

I see no reason for a rewrite based on those problems. You have a codebase that has been working for 18 years, and still works. The problems you described are with maintaining it, and usability, but not with its functionality.

There is no documentation, so write some. Question decisions were made, so do (major) refactors to correct those decisions. Likewise, you can pick pieces to modernize, one at a time, and improve the UX one piece at a time.

This does sound like a sizable project to get it where you want, and if you do one piece at a time, you may end up with 90+% new code when all is said and done anyway... but you'll do it in bite size pieces without impacting your customers or your stability.