"Three wooden privies to the Northeast are foul and inadequately maintained. Two are presently occupied"
Any cult that can't keep their privies working are definitely not worth joining!
There's a lot to like here- cultists and prisoners, a giant idol, a monster that will happily eat everyone that comes through a portal, mushy organs and one of my favourite beasties, the gibbering mouther, who is always good for encouraging screams from the PCs!
Other posters have already made the points about the design of the adventure. What I would say is that this is a classic style of D&D game, so I wouldn't change the plot, just give it room to breath.
And draw a big old Gibbering Mouther for the first page, it'll set the adventure up a treat!