I beat the whole thing and I noticed that at least right now when you get to the end, the special squimos stop spawning that you need for advancing research. But I imagine that is just a quirk of the demo mainly.
I beat it yesterday before you added the ability to switch back to previous stage, so probably this issue doesn't matter now.
Sorry, the fact that elite squimos stop spawning was actually an intentional design choice. I wanted research to be tied to value and a sense of achievement, so returning to previous stages does not allow players to farm them again. However, I'll reconsider this design, and I may need more feedback to find a better balance.