The sunk cost fallacy almost pays off as this is the best of the bunch.
With the cast of the first two parts playing inhabitants of the 17th century settlement "Union" before it became Shadyside and Sunnyvale we get teen rebellious partying, some lipstick lesbianism and some truly awful "Irish descended peasant" accents. When Union is struck by pestilence and madness, the self righteous look for scapegoats amongst the sinful youth.
It still suffers from the "children's book for adults" factor but there are fewer jarring bits, the story at the heart of it is decent and the 17th century sections are blissfully missing the pop culture inserts and soundrack.
Eventually events flip back to 1994 and there's a pretty satisfying showdown.
Overall this series just wasn't for me. I really feel it's too adult for kids and too childish for young adults and I am now very very over retro pop culture insertion to the extent it irritates the hell out of me.
This makes me suspect I'll hate the next Stranger Things when I've loved the previous ones. Which would be a shame. 5/10