Are the given/prepared options the only ones? That is, are the players picking from a menu, even if an extensive one, or are they "in the hotseat" and actually creating story beats? AIUI, that's the gap between adaptive "Story Before" and actual "Story Now": the difference between a standard buffet of pre-cooked food and actually participating in the cooking process. One might call it "fulfilling" vs "creating." If the players are simply fulfilling what story opportunities you offer them, that might cause issues.
The only given option is the setting location. Players needed to agree that we are going to play Rappan Athuk and have a reason for their characters to want to go into the dungeon.
Rappan Athuk is a 500-page beast of a mega dungeon with over 56 dungeon levels and several satellite dungeons with another 20 levels. There are over 100 keyed and color-coded maps. 22 wilderness areas. The timeline is pages long. There are many, many groups, factions, and important NPCs. One thing many people hate about the Frog God Games is the walls of text of lore and background information but I enjoy it. There is no menu to pick from. Find a way into the dungeon, explore, see where you end up and what happens. Based on the players actions and decisions, I'll decide how different groups react and I start writing up new plot lines.
We do keep a quest log in our VTT, that is kind of menu. Some are quests given by NPCs or from rumors. But many are quests that the players came up with--basically objectives they want to accomplish.
E.g. you said "a lot of potential plots and quests," implying picking from a prepared menu. It may be a big menu, but it's still prepared. Some "modern-style" players will chafe under those limits. Or you may feel run ragged when the players ignore every menu item and start creating brand-new dishes they want to eat.
Well, having a map is a menu. Having NPCs is a menu. This is the most player driven campaign I've ever run in D&D. If this campaign would chafe players, I wouldn't run D&D, I would run InSPECTREs or Dialect or some other system where the world building and story evolution is heavily player driven in an improvisational, yes-and, theater of the mind manner.
In my campaign the dungeon is the constraint rather than a plot. An in my experience playing in Adventurer's League games, players are more uncomfortable with too much open-endedness. One thing I noticed about all the AL games I've played in is how limited and linear many are and how many players get annoyed with players who try to go in a different direction than where the adventure is obviously pointing the party to go.
Yes, in my current campaign, you are "stuck" with the dungeon. In my first campaign for 5e, I created a huge and detailed homebrew world. And I ran it is as much like a sand box as I could, but there was SO MUCH the players could choose to go that I found it unsatisfying to completely improvise. So I ended up asking the players at the end of the session what they would like to do next, where they would like to go, so I could prep something more satisfying. I know some no-prep DMs can run games completely by the seats of the pants making everything up as they go. I'm not one of them. I find it exhausting and I would rather do some prep work than have a bunch of homework after the game trying to write up notes to have some sense of continuity. For a one-shot game of InSPECTREs, it is fun. For a years-long campaign? No thanks.
In a way, a mega dungeon is very rail-roady. All the maps are made and prepped on the VTT and its populated with monsters, factions, traps, and puzzles. But the story itself is created by what the characters do in this environment. They decide why they are their and what their objectives are. It's not for everyone, but we've enjoyed it enough to play 8 or more hours a week in it for over three years.
But perhaps this is just a poor turn of phrase. Do your players get opportunities at actually creating story themselves, rather than fulfilling a story-hook you provided them?
The only story hook is that there is a legendary dungeon surrounded by dark rumors in the wilderness. They players only have to agree that they are interested in checking it out. Their reasons for doing so, and what they do when they get in there, are entirely up to them.