A major problem with Finn's character

It's an action film not a novel written by Jane Austen. If you want in depth character development try Period Drama.

Wrong, the defection was just the beginning of his character arc, not the end. His real arc was to find himself as a free man and to become the good man he had to the right to be, and to find a place in the world and find love.

What a pity the writers forgot that, I liked him.

To be fair, they might have been considering his arc to be that in the beginning, all he wanted to do was escape the First Order and go somewhere that they couldn't find him. But he began to care too much about the rest of the people, so eventually decided to stick around and fight the bad guys (pretty much identical to Han Solo in the original).

Its not a finn standalone movie, its a sequel , this is just nit picking for the sake of it.

Boyega was pretty pissed off about it. He even cried racism because of the lame character development. They turned him into a black servant constantly sticking his neck out for Rey, even though she's a lot more powerful and resourceful than he is from the get go, and particularly as the movie progresses. He is basically chaperoning her and being infatuated by her. But he never gets to hump that sweet ass, which would vindicate his efforts. Kathleentians meticulously picked a diverse cast, but kept the relationships between them very old-fashioned.

Screenwriter 1: I dunno about this Finn guy, I feel I need to give him a stronger motivation.
Screenwriter 2: Ah, just make him scream "Rey!!!!" all the time.
Screenwriter 1: OK.

Yea but so is everything.

We have no idea nor care about Kylo, Rey, any character except maybe Han, and Leia. We even have Max Von an outstanding actor killed within a few seconds of the movie before we can have any sympathy for him.
The Novel is quite different. We are supposed to have a sympathetic character in Kylo Ren but instead we get this crazy raging lunatic.