A megadungeon needs a committed group and a committed GM. One without the other results in a failed campaign after a few sessions.
In my experience, megadungeons work better for video games than at the game table.
As with any campaign, talk to your players upfront and get serious buy-in before doing the insane amount of work to create the megadungeon.
Personally, I think 6 levels with 20 areas per level is all you need to run a 6 month campaign of weekly 4 hour sessions and that would be a reasonable amount of work for a reasonable campaign attention length for the modern game table.
Or 12 levels with 10 areas each if you want to go deeper, but still run a 1st to 10th level campaign where the players feel they are making progress.
And when I say "areas", I mean meaningful zones, not empty rooms for spacing / travel / wanderers.
For my last megadungeon, I ran Tunnels & Trolls and did 6 levels with 6 zones each and we played about a dozen sessions to clear it out and face the Boss monster (who ate the party).