Hi! I was able to get Jamie to ride the slide with the MC, if you want some advice for how to do it below.
So it seems like a lot of the choices you make affect some kind of point counter for each of the love interests, and the higher the point counter, the closer your MC is to that LI, which unlocks some unique interactions when you get enough points. Jamie is sooo hard to get close to tho lmao compared to the other interests, I had to make a playthrough just for him.
What I did was:
1. Always choose Jamie if given the choice to choose him (i.e. choose the petting zoo in the spring festival after splitting up). When Jamie wasn't a choice, I chose one of the besties instead.
2. Make sure not to choose options that could affect his perception of MC negatively (i.e. DON'T fail the skill check with the volunteers in the kitchen during the barbeque event or keep staring at him in the study event IF you don't have enough points yet).
3. Finally, I made sure the rainy day event was my last played event so that I had the most points possible.
Not only was I able to get him to slide down with MC, there's some unique dialogue when you walk with him to make the leaf boats! Alternatively, there's unique interactions for the other events when you get close enough to him (one of my favs is a hidden choice when you get the tattoos in the spring festival event).
Good luck! :)