How many more times do I have to repeat myself?
Shinzon's motivations are all about Reman nationalism: He wants to liberate the Remans and is against the Romulans.
Keep repeating yourself, it doesn't change what we all see in the movie.
You are correct. Shinzon's motivations were about liberating the Remans and opposing the Romulans. That part of the story still occured, it was the opening scene.
He succeeded. He won.
The rest of the movie is about his craziness AFTER that.
"I defeated Romulas, now I defeat earth!" He's just defeating them differently. Everything you want is right there. You just seem upset that the part you find most interesting was over in 3 minutes.