... zero on anything beyond that. AFAIK, there's very little related to any of the other adventures in the module. ...
Well, No worries. The only catch is the "AFAIK".
But, there's nothing wrong with that. Not knowing is simply not having gotten information, yet. But, I can help there.
So, one can search names of individual adventures, and there's a slew of stuff.
There's even
DMsGuild Silver,
Gold,
Platinum and
Mithral best-sellers. There are ones for individually named adventures. A
great deal of stuff for the "Ghosts of Saltmarsh" campaign as a whole that's not specifically for the town of Saltmarsh—like magic items, NPCs, and even maps.
For places like the Ghost Tower of Inverness, Isle of the Abbey,
Whole books about diseases, NPC full page stats, digital assets for digital tabletops, hideouts, side quests, and even other stuff like the Scarlet Brotherhood and other (some new) factions.
There's a
lot of content for all distinct aspects of GoS which have nothing to do with the town of Saltmarsh.