Really enjoyed the game! The atmosphere/mood/theme was definitely on point. I like how you tied most things to the buttons, made it very easy to grasp what is going on. Also props to you for getting a whole settings screen in there and bless you for including an actual menu system.
When I got to the 3rd island, I didn't know that I had to use the moving platform to get to the 4th one. Maybe, turning the camera to the "thing" that just activated could help, but I'm not sure how you'd do that while the island is dropping...
I managed to find endings 1 - 3, but I have absolutely no clue where to find endings 4 & 5. I thought that maybe each island would be associated with one ending, but I couldn't find any buttons on island #2 & island #4 Maybe some kind of hint at the end of each ending could help players find out the next ending? Or did I miss the hints? You could also try to hide the buttons on island #3 or #4 a bit more to ease players into the idea that they'll have to search for hidden buttons?
The game reminded me a lot of Superliminal & Antichamber, and I thought that some kind of narrative aspect could really help tie the game together (of course that's a little bit demanding for 72 hours). That way, I feel like you could really provide some meaning to sequences such as the one where "I keep pressing the button but the island keeps moving away".
Regardless, my feedback is a little skewed because I'm not a big puzzle person. You might be able to get more valuable feedback from folks over in https://discord.com/invite/eybtCDR (I've heard good things about the discord, especially if you're into play/making puzzle games).