After reading Chapter Four, I'm realising that your story would be better without the Utlra wormhole subplot. It feels kind of tacked on and unnecessary at this point for a story that seems meant to be focused around Grimsley and Nanu's personal growth. Maybe this is just me personally but I'm enjoying Nanu and Grimsley's story a lot without the game plot interfering with it.
You were also right that the chapter was too long. It would have been better to do the desert as a separate chapter to the rest. I will continue to praise you on how you handle characterisation though and also on the description of the desert because that drew me in and felt very real. I could practically taste the sand myself.