Yeah, too bad Jellyfin doesnt have a way to parse a DVD and make menus. That way, I could be a better archivalist.
Normally I just convert the largest title too the name of the movie, and put the rest in extras, so Jelly can pick all the extra content up.
This is a thing I want to try and do. I do the same thing as you are the moment (mainly to convert things to the much more efficient mp4 instead of what you natively get from, e.g., MakeMKV) just throw the movie in as a single file, and then all the rest into appropriately named extras.
But I do try and save the (usually pretty small) menus just in case I can figure out how to get them running at some point. VLC can run the menu with the large videos stripped out. I still need to check, but I assume I could create a single iso file that would run as a "menu." I guess I'll see.
I worked on this problem a little bit. Unfortunately, it's not really a matter of simple "parsing" - the DVD menu is really a program with an entire virtual machine (IMO overly confusing and complicated).
You basically have to implement the VM, which may lead to legal problems (?)
But, VLC does this (via libdvdnav and libdvdread), so I'm not sure why more don't.
I've also been wishing Jellyfin has this. I've not tested it myself but a friend shared this plex competitor which apparently supports DVD menus: https://ampchor.com/
some software could just open a DVD ISO, so I used that as the backup format when I wanted to retain menus. XBMC and it's derivatives could open them over the network share iirc.
VLC can do DVD menus as well, either from an ISO or a VIDEO_TS folder.
Kodi can remotely play DVD ISOs. It does mean you get no compression though.