i think this is a creative use of the title and a solid premise. Eaten by a giant monster/being inside larger creature is a time-honored trope that gives you a lot to work with. I think the shrimp people are fun and funny. Are they parasitic/symbiotic with the Zetafish? RT1 should have more detail about what the players could interact with. Describe some aspects of the Zetafish so GMs have something to work with. How is it different than other space whales we've seen in other media?
I would advise against calling rooms "boring". You've got total control here. Don't make things boring, make them interesting. If there's nothing to do, there should be something to learn. Maybe the villi/phalanges have evidence of other things the zetafish ate. Maybe there's loot there. Maybe there's evidence of some high tech/magic species the PCs have been looking for. I don't know, but don't call stuff boring.
This has the structure of a fun, unexpected adventure. I wish there were more details on the Zetafish ecology and what we find there, and less mechanics and GM asides. I think you're almost there.