I’m in the same boat as you, unfortunately. I would love to find a way to implement a DVD menu or subtitles to the DVD copies of Harmy’s Despecialized Editions, but I’m afraid that I don’t know a way to go about it. I plan on making a set of Despecialized Edition DVDs for my aunt, as she recently expressed interest in re-watching Star Wars but hasn’t seen any of the films since they were originally released. I really would like her to be able to watch the versions she remembers seeing in theaters with the option of subtitles, should she need them.
As for making a menu, I’ve wondered about this myself. Aside from the now-defunct Adobe Encore, I don’t know of many other DVD/Blu-ray authoring software that people have recommended. I’ve heard mixed things about Vegas DVD Architect, but their page is here if you want it: https://www.vegascreativesoftware.com/us/dvd-architect/ . It’s a bit pricey, but it handles both DVD and Blu-ray authoring and menu creation.
Hope you get a good lead on things soon.