SPECULATION Luke as the generational pivot between the light and dark sides

I don't see how Luke being a secondary character can lead fans to any conclusions regarding his or Kylo's fate. We can speculate that he will be a mentor for Kylo if he's willing to listen, but that's pretty much it. Luke helped in the redemption of Anakin and he can help in Kylo's redemption too, and doing so as a secondary character is more than enough given that this is not his story anymore. Are you saying that Luke is in a way purposeless in this trilogy because he no longer has very close ties to a main villain? Because Luke could have been great in the original trilogy even without Vader. His father's redemption was only an element of his story, not its purpose. He was on a hero's journey, the possibilities for character development were endless. And the same is the case when it comes to Kylo's redemption. He can achieve it without Luke, his possibilities are endless too. So again, Luke's role in this story says nothing about his or Kylo's future.

Probably not. He was the hero of the original trilogy, so turning him into a tragic figure would in a way deny the message of those movies. Not to mention the fan rage that would follow.

It would say the same thing Vader's redemption did. Except Kylo Ren didn't cut his son's hand, didn't choke his pregnant wife almost killing his unborn children, didn't torture his daughter to the point where she thought her heart was going to explode, and didn't cut down a temple full of children while the adult Jedi were being assassinated by stormtroopers.
But I know what you mean. Redemption seems worthless compared to the crimes committed by both Vader and his nephew. Still, this story is an allegory meant to teach children (and adults) that it's never too late to change your ways, that the lost are always welcome back home. Even if we wanted to judge the characters in the same way we would real life killers and torturers, we couldn't, for the simple fact that they're influenced by something we don't understand: the force. We have no idea what it does to people in SW, so little of it is explained. SW characters simply can't be compared to real people.
But if we really have to do that, let's think of it like this: some kid is born with a power that is understood by a select number of people. Then that kid is isolated and corrupted by an evil sociopath and ends up committing horrible crimes. In the end he understands just how wrong he was and escapes the influence of the dark side of this power. How do we judge him? We have no idea what it's like to be him. We have no idea how his mind or body works under the influence of that power. This person would be alien to us. The only people who can judge him are those who understand his power. And before someone says "Well, we don't understand how the minds of sociopaths work but we put them on trial anyway", no, it's not the same thing. We do know how their minds work even though we don't know what it's like to be them. But when it comes to people that are influenced by something that can be used to throw planes at other people... no, we would have no idea how their minds work and how much free will they actually have.
So yeah, redemption in SW = positive, heartwarming message.