I've said before that while I understand (narratively) why Simon died I personally would've gone about it differently.
After Grace is saved by the paper cranes the ghom just decides to fuck off instead of attacking Simon, he sits there as Grace is embraced by the the soon to be former Apex and how close he came to losing what was likely his closest and only friend in years. Obviously this doesn't get him to do a complete 180 on his beliefs (because it doesn't work like that.) But maybe, just maybe, he starts to think about how far he's gone.
And it's not like he or Grace can go back to the way things were as the Apex, not after a good chunk of the kids witness the fallout between the two co-leaders (not to mention the attempted murder like these kids can NEVER see either of those two the same again.)
I haven't exactly decided what exactly happens between this and Simon leaving but honestly I don't think they'd have much to say to each other after all that. Either way Simon leaves without saying a word to Grace or the other kids. It's not as bad as him straight up dying in front of Grace but still very devastating that she's yet again losing someone she considered a friend / someone she cared about.
After that... idk maybe he'd go back to the cat / Samantha? I think they have a lot of things to work out together. It'll definitely be like pulling teeth with those two, and it won't completely make up for the trauma Simon endured as a child because of Samantha's negligence, but they have to start somewhere. (and yknow... maybe acknowledge the whole thing with invading Grace's memories and trapping her in her own tape.)