I really like your idea of an Imperial Triumvirate. I would definitely run with that idea. It would provide a lot of potential for factional infighting and intrigue, while allowing the IR to be more multifaceted.

Although I liked the Imperial Knights, I always perceived them as a neutral entity who have no quarrel with the Jedi, which defeats the purpose of being antagonists, so I am sticking to my version of the Knights of Ren even if they are merely “Sith lite” as you put it.

I suppose so. But you could still make them fit a more antagonistic role through their political alignment. Simply, the Jedi would have to oppose them because they’re the enforcing arm of a tyrannical regime, even if they don’t use the Dark Side. They would represent what the Jedi Order could become if it were led astray by a charismatic leader with rhetoric about Force supremacy. So, the battle between the Jedi and Knights of Ren would really be a battle over the future of the Light Side and its role in the galaxy. Though, you can always merge both ideas, and have Kylo and the Knights of Ren gradually slip further and further into the Dark Side as the war progresses.

Actually that would make Luke a man in denial, which IMO would be far too depressing.

It’s not denial. It’s just Luke doing what he’s always done: carrying on in spite of trauma and adversity. It would endear the audience to Luke, showing the very human struggle he’s going through in his life, while also showing that he never gave up on his duty. It’s more realistic to how depression works in real life, and it doesn’t take away Luke’s likability.