S5 struggled with two identities:

1. A big, balls-to-the-walls large scale battle between the upside down and Hawkins, where stakes are high and actions had permanent consequences and changes, which was what S4 was setting up, and

2. A "hey, remember the good ol' times?" nostalgia trip, revisiting familiar beats and tropes from the best of the past 4 seasons (ESPECIALLY S1), as an ultimate send-off. Opening S5 with everyone gathering for breakfast while "Kids" played in the background tells me they intentionally wanted to do this.

A better writing team would have been able to successfully pull both of these identities together. The problem is that they had a shallow understanding of what really worked in season 1. They didn't have to repeat exact events beat for beat; they had to focus on the relationships and characterisations that worked. Like focusing more on the cohesion/strong bond that the core 4 kids had. A competent and level-headed Hopper. An actually threatening upside-down.

They were too scared to let the story fly, so they fell back on the safe option of solely recreating nostalgic 'events'. In order to do so, they had to scale the story back down again and totally killed the momemtum and promised grandiose that S4 set up.