The problem with the ToEE is not that it's a big dungeon crawl. I love the
big dungeon crawl concept, and I like
creating and running them.. The problem with ToEE is that it tends to be a boring slog of a dungeon crawl, if run as written.
The first issue is that ToEE is what I would call a "lair style" dungeon writ large. That is, it has a more logical and purposeful design compared to a magical madhouse underworld. That "realistic" ecology and reasonableness makes the classic mode of adventure in a megadungeon (i.e. exploration of the unknown) hard to pull off. The way ToEE is set up, you tend to end up facing organized resistance to incursion that makes for a lot of similar encounters with the dominant groups (e.g. another room of gnolls or bugbears or whatever). In a place the size of the ToEE dungeons, that makes for a slog.
There also isn't very much empty or dead space in the temple dungeons. That contributes to the "go to the next room, fight the humanoids" factor. If there were more "no man's land" areas in the dungeons, it might open up the exploration aspect, and also perhaps give more opportunity for maneuvering among the temple factions.
The temple factions are the key to making ToEE more fun, in my opinion. Instead of relying on an exploration-based approach (i.e. explore and fight your way through the dungeon), which ToEE isn't well suited for, you can spin it into a more "political" adventure where the focus isn't on exploring the dungeon as much as it is on chicanery and negotiation and playing one side off against the other. In order to do that well, you need to really work up the NPCs for the various factions, and provide lots of opportunity for the PCs to get involved in that fashion.
With that kind of approach, forays into the dungeon might be more commando-style raids with assistance (maps, hints, or even escort through certain areas) from dubious allies, rather than traditional "explore and loot" expeditions. And there would probably be as much game-play and action outside the dungeon as inside. Nulb would probably be extremely well-developed by the time such a campaign ended.