Overall, I like your rewrite ideas. Your version of the Imperial Remnant sounds good, and I like what you’re going for with the Knights of Ren and their whole Force Supremacist ideology, but I’d suggest finding additional ways to differentiate this new ideology from the Sith. As they’re described now, they sound like Sith Lite, which is okay, but I think could be made more interesting. One idea (I’m not sure if this is what you were thinking) would be to make the Knights of Ren similar to the Imperial Knights from the Legacy Era: Force users who reject the Dark Side, but serve an authoritarian regime, perhaps out of some sense of a “greater good.” That way, the story won’t require bringing the Dark Side back after RotJ, but will portray a whole new, different corruption of the Jedi ideology that has to be defeated. I wouldn’t have them massacre Luke’s Jedi Order, but I would have them convert most of Luke’s students away from his teachings, leaving Luke disheartened but not totally broken.
I don’t agree with having Kylo Ren as the leader of the IR, though. He can be the leader of the Knights of Ren, but the IR’s leader should be someone the Knights swear fealty to. A Force sensitive, also, but someone older and with more military bearing. Kylo could be basically the chief lieutenant/enforcer of this new Emperor.
As for the Imperial Remnant’s technology, I like the idea of more self-sufficient TIE Fighters, but I’m still not on board with the Death Star lasers coming back. The IR’s strength should be through subterfuge and surgical strikes, like you described. So it would be better if they didn’t have world-wrecking technology to fall back on. Their strength would come from their tactical cunning instead of raw firepower. One thing I would be careful to do is to never show the IR’s military power as overwhelming or insurmountable like the Imperial Fleet was. With battle scenes, there should be the sense that the odds are more even now, and that the IR can sometimes lose in pitched battles that the Empire would have easily won. If done well, it could create more tension, because the audience by now is used to seeing underdogs beating the stronger foe, but when the situation is closer to 50-50, it won’t always be clear what direction a battle will take, and you can use that unpredictability to your advantage.
As for Han, I’m fine with him being written out, for Harrison Ford’s sake. Though I would like for Han and Leia to have a kid or kids. They wouldn’t have to be Force sensitive, and they can just be minor characters,
I like your plot outlines, but I would try to flesh out VIII’s climax a bit more. The part of Kylo and the whole IR fleet leaving Coruscant, while a good character moment for Kylo, seems too abrupt from what you described.
Luke being depressed is fine, but I would rather have Luke’s depression be depicted in a more realistic way. The vast majority of depressed people don’t behave like ST Luke does. They still carry on with their lives, struggling day-to-day while trying to retain some degree of normality. The signs of depression are usually more subtle and easily missed by people. You could have Luke still trying to train his remaining students, but it’s clear he’s lost his old enthusiasm. His room is messier. He seems tired a lot more. Little things like that would go a long way.