In my view, Kylo's redemption arc starts at the end of
TLJ when Luke shows him how foolish and useless his anger is, and how truly alone it's made him. It's Luke's final lesson to Ben.
And then in
TROS, it's driven by the return of the Emperor which sweeps the rug under his feet and really makes him realize how much of his life has been a lie. Snoke, his grandfather speaking to him. He has a kind of rebirth when Rey stabs him and stops him from dying, and then is finally able to find forgiveness when Leia shows him the memory of his father. Fighting and defeating the Knights of Ren and facing Palpatine is him finally confronting and turning his back on all the manipulation. He dies giving his life for the one person left that he loves. It works for me.
A lot of people misinterpret the ending of
TLJ as cementing Ben being evil, when really it's the opposite. He starts the movie kneeling to Snoke. Then he achieves his major goals to gain power - Luke is dead, he kills Snoke, and he's the Supreme Leader. And yet he ends the movie without any satisfaction. Kneeling. The same way he began.
So yes, Colin's version would have been many times worse.