Massive spoilers. This is just a rough summary to get things out of my head.
Season 1 is about Eleanor getting sent to the good place by accident. She spends the season trying to be a good person and actually belong in the good place. At the end they find out this is the bad place designed so that the humans torment themselves.
Season 2 is about Michael rebooting the system constantly as the 4 humans keep becoming better people. At the end of this season Michael teams up with the humans and becomes a good person as well. He realizes the system is broken and helps the group see the judge to see if they can be saved.
Season 3 is about the humans doing a redo on Earth and seeing if they can become good if they survive their near death experience. This season ends with Michael realizing the entire system is broken and that everything needs to be investigated. This is when he learns that its not that the bad place demons hacked the system but that life on Earth has become so complicated that no one can be good.
Season 4 is about testing if humans can be good in an environment that isn't complicated. They run an experiment where now Eleanor is the architect and 4 new humans get put through what the original 4 went through. The season ends with them proving the system is flawed and humans can get better. The entire system is redone which Michael in charge of the good place.
The final two episodes are stupid good. The total ennui of the good place is making everything dull and so they allow even the good place residents to truly die. The final episode is about accepting this from our main characters.
Extremely depressing end and something to think about. I get the emptiness feeling that I get when you spend time with somebody and then they leave. That hole in the heart feeling. Fuck.