
I wanted them to kiss sloppy style, but instead we got age regression....sight

I think the author might keep them platonic for a while or maybe forever. I don't really mind, they can be found family that's a little gay, why not.
I just wished they'd gotten closer by talking and bonding over their near death experiences instead of magical age regression forcing Sayo to look into Yamadera's past.
With the direction that the story's taking I might drop as I don't believe it is really developing the relationship or the narrative very well, but I'll wait for the official translations and re-read it.

Okay that i agree with.
The magical "he's younger mentaly now" needs more explanation or it's just coming out of nowhere
(on another completely unrelated note, they already love each other, it's clear, whether that's platonic or romantic, that i don't know/care, but i'm glad they already have a relationship (however wonky it is at the moment i'm sure yamadera'll get better in the end))
How many people complaining about this manga have read smut fanfic/doujin about 16 year olds before...
A lot of anime and manga are centered around teenagers, and many are the BLs set in schools, so I bet at least 50%. So why is noone complaining about those 16 year olds having sex?
If it's the age difference that bothers you, you're probably too young to understand that there isn't a shift in mentality once you reach 18; you still are a stupid teenager.
If you're going to be all moralistic about 2D porn, try to be consistent about your values.