Your entire review is one big spoiler. LOL But that's okay, because as you say, most are already familiar with the basic plot.
I agree that Sim plays Ebenezer to a tee. I think he goes deeper with the role than other actors. Maybe that's because with the backstory more developed, this script allows him to go deeper.
Sim had a prolific career on stage and in films. He played all kinds of roles. Some of his comic parts are almost better than him reciting all this humbug. You should check out a few of his other films.
I don't know if the Ghost of Christmas Future needs to be as scary as you seem to want him to be. This is not a horror film, and in fact, I think some other versions go too far with the grim aspects of the Future. This production veers from going overboard. Plus if you think about it, he has already begun experiencing an epiphany in the other two vignettes, so his future is looking a bit brighter now, not so horrific.