About all of the boys being on bikes and that being a callback to s1 — I made a post some time ago about how Will’s bike (and its presence both on the show and on posters) symbolize his disappearance and trauma, and I still agree, but I wonder if the bike on the poster can also mean something else:
Childhood (how things were like before)
Season 1 - all kids on their bikes
Season 2 - all kids on their bikes
Season 3 - no one on their bikes (but Will has his bike next to him, and there’s a bike between Lucas and Mike).
Season 4 - no one on their bikes (but Will has his bike next to him)
Season 5 - all kids on their bikes
Season 1-2 everyone was riding bikes.
But in s3, where they “aren’t kids anymore” and are “much too old for dnd” and focusing on acting older and are ignoring their friends, they suddenly aren’t. Except Will has his bike, Will wants to ride again, to have things go back to how they were before, to still be a kid. We know this, yet there’s also another bike between Lucas an Mike. They actually want this as well, even if they pretend they don’t.
By season 4, Will still holds on to it, even if Mike is suddenly bike-less, perhaps accepting than things will never go back to how they were before (or will they) (smth smth love confession).
In s5 they are all going back, they’re all working together, they’re all spending time together (and probably playing dnd), they’re all prioritizing their friendships and are done acting mature. We’re back to s1 and s2.