Eh, Shinzon is still too wonky for me. He may indeed have multiple motives and varied goals, but his actions within the scope of the film are confounding.
His most immediate motive should be to save his own ass--the bit about needing the blood transfusion from Picard or else Shinzon will succumb within days or hours.
With his very life in immediate danger, you'd think Shinzon's main goal (at least in terms of the movie's timespan) would be to secure Picard to get his blood.
Instead, he delays and distracts, with android scavenger hunts and otherwise jeopardizing Picard's (and his own) life needlessly.
Then Shinzon talks about liberating the Remans, he calls it "the single thought behind everything [he has] done". So that's his stated motive.
I suppose part of that includes killing off Romulans, because he takes quite a bit of time executing that plan.
Except he also has the personal grudge against Picard, where Picard must die so Shinzon's existence can have meaning. That becomes Shinzon's obsession.
But before Shinzon tackles these matters, he's got time (with his life counting down to hours remaining) for some mind-rape antics.
Evidently all this is not enough on his plate, so Shinzon throws in the destruction of the Enterprise and everyone aboard, and then continuing on to also destroy Earth, too.
It's just way too muddled and needlessly complex for one script. If they wanted the personal obsession/vendetta against Picard, that would have been fine. Or the Reman revolt for freedom. Or the plot to destroy Earth. Or revenge against Romulus. Or the quest to save his own hide with the blood transfusion.
But all of it, in a jumble? Maybe they were trying for a complex villain or a multi-dimensional character, but it seemed to be a mess to me.