I'm... exactly sure if I agree with some of the stuff you mentioned? I don't think a threat needs to be bigger than the previous one for example. I have this belief where I think it should fluctuate? Let me explain. Jojo Part 1 was about this vampire. Part 2 was about THE ultimate being (who died by dumb luck). Part 3 was about this vampire again, except he has a cult with connections to all sorts of crazy powerful people that makes the ultimate being look like nothing. What did the author do with Part 4? He made it about a murderer who wants nothing more but to have a simple life in a small town... while happily killing people.
He doesn't want to move out... whenever he's caught, he blows up the protagonists, goes back and time, and things reset... meaning the protagonists don't exactly die. Part 4 sorta plays with the expectations that the next villain should be something big and deadly. Akira Toriyama has this thing where he tries to raise the ceiling with every villain. It's like a Shonen trope. Araki plays around with this by exploring the morality of the heroes. Like what do you do with a villain that can get away with murder and wants nothing more but to stay confined in this little town?
The writer establishes that Heaven and Hell exists in this universes and doubles down on it in this arc, so you can let him grow old and allow him to burn in Hell while the victims go to Heaven. It's like the author is pushing Shonen character's sense of judgement to its limits while keeping the scale as small as possible (Japanese people LOVE characters with strong sense of judgement). The funny part is even in the end the DUDE GETS AWAY WITH IT. He gets pulled into this spirit world, but he kills people as a ghost assassin... the irony is he's doing it out of obligation and he has amnesia (and they hint that he could get his memory back). Still tho, in the living, the heroes managed to prevail despite the victory being super small in the grand scheme of things.
I think you can do a small threat, then maybe do it as a deconstruction or maybe... make it super interesting. Either way, if you just do whatever with threats instead of making them bigger each time, then you have an excuse to return and do something new. It also makes the protagonists seem a little more special? Like "Yeah, I saved the universe in this entry, but I also did this in this small town."
As for characters relearning their lesson I'm actually fine with that. Especially with Guardians where Rocket was stubborn. You think he learned his lesson in 1, but no he's stubborn and in 2 he's still a stubborn jerk but yeah... it makes sense. He's a stubborn jerk. I feel like writers should embrace this kinda stuff since humans are hardheaded in real life and change is a process.
Regarding romances, I think the sad thing is that sometimes it's never happily never after behind the curtains. Romance is a complicated thing. I'm okay with the character restarting relationships and just never learning because some people just don't learn lmao. I remember in Macross: Do You Remember Love where these two characters get married and have a baby after falling in love for like a day, being the first interspecies couple (it felt fairy tale like).... but in Macross 7 it's revealed that the two got a divorce and their daughter is just socially awkward as a result. I don't think... they get back together... but it's interesting that they're still the heroes of the story.
And then there's arcs where a character just doesn't WANT to learn. Like in Jak X, they reveal that Jak has a habit of leaving people for dead and when you look back at the series... yeah... he does. Originally, this was an oversight by the writers, but they were like "screw it, the dude gets a kick out of it" in the final entry despite there being a scene where a villain's daughter is heartbroken by this.
As for Thor (and I'm going to go full comic book nerd here), y'see.. Thanos is in love with Death. He isn't anymore in the movies, although the films do this cool thing where they have death be Thor's downfall. In Thor: Ragnarok, Thor's much more cockier than usual which makes sense. He's been hanging around with the Avengers. The villain is symbolic. She's the Goddess of Death (eh, ehhhh?). After Thor beats her and unleashes his godlike side, he feels like he can face anything... until Thanos comes along. And then it's like this twisted version of the original Thor movie where he has to find himself again, except now he becomes this fat nobody except for this nobody who happens to be handsome.
It's like in real life where you think you can handle everything and you got life in order, but then you realize "Oh shoot. I'm not strong enough after all...". But then you push yourself to become something better. Nothing wrong with reusing arcs.
Although you are correct that it depends on execution.