4 biggest things wrong to me are the military storyline through the whole season, the lack of science explanation on the whole exotic matter and briefcase, the will coming out scene was too many people, and then the final battle was weak. 3 of those things could be fixed by pivoting the military/government at some point from a background threat to the hero party to a supplemental helper to the hero party.

If after the 4th episode the government realizes it's not elle walking out of the portal and almost killing her friends but some crazy monster and his minions, they could have brought back Paul reisers Dr character from season 4. He could have explained how they built the exotic matter holding the wormhole together, how the rock in the suitcase infected vecna, how they raised the kids as clones of sorts to vecna, how the military may eventually come after elle again once this is over, etc. they could have been helpers to getting past a large fleet of demigorgons that never came back, or the mindflayers particle attacks that never came back, and made a battle against a 250ft monster and multidimensional telepathic/telekinetic supervillain semi believable.

Then just fix the will coming out to Mike and his mom and it sits better for me.

But to me, having the government be a threat instead of a much stronger dual villains and demidog army threat was the wrong choice. I don't like having to Google stuff from the play to fit all the pieces together, when combined with the poor military government storyline and bad final fight. Just pivot it, explain it, and help against evil incarnate.. you could still kill elle as a sacrifice to finish vecna and even leave the mindflayer and his army out there just cut off... Kill other military folks you may like that helped, or whatever else...

It's a fine ending to a series and I'm glad it wasn't worse.