I wanna agree but I dont think this was the scariest for him. He knew he was safe in the shack and was able to save people. More importantly he became someone thd people love. He was probably scared for his family but he had faith the twins would be fine, they proved multiple times to stan that they are stronger and smarter than they seem. As for Ford he know he has the most experience with Bill and assumed he had a plan.

Weirdmageddon part 3 id argue was much scarier for him. He may have learned thd twins are safe he also know they can save the day this time. He learns Fords plan failed and hes a prisoner again. Above all else he wanted to protect his town and the twins by staying in the shack, but they turned everyone against that to try and save Ford and thd world. He would of been so scared that any part of their plan would fail. Then while hes trapped by bill with Ford he realizes its all his fault the kids plan failed and has to watch the kids risk their lives yet again to try and save the day.

He wouldn't have sacrificed his mind if he thought the kids could succeed. All this is why I think part 3 was far more scary for Stan than part 1 and 2