To tie into that lore & create an interesting subterranean transit system you could somewhat mimic what you’ve done with Forthos by having an actual shortcut/entrance to the catacombs within each of these newly split up dungeons. So they all still technically link up to one another in a round about way while being spread around the map a bit more.
There are a few old dungeons that do that which are quite neat, even if they're not often used (like the KQ hive being accessible from south of Dorgesh-Kaan & an area in Waterbirth dungeon leading to the lighthouse dags).
Edit: Nevermind, just saw your other comment mentioning this kind of thing already, so that's cool. :)
I like that we’re on the same wave length :P