It’s an interesting idea.
One change I would make would be to make the eyes of the original actors match their younger selves. As people age their eyes lose some shine, it’s usually the tell in even really good old age makeup. So if you made Carrie, Harrison and Mark look like they are wearing old age make up rather than being natural it would go some of the way to pulling off the effect.
I would also make all the original class limp a bit because that’s what young actors think older people do.
I’d replace most of Snoke with the original Palpatine (mask sections and animal eyes) just keep the lips for sync reasons and maybe use computer morphing which would be a cutting edge technology at that point.