I've only seen the movie twice, and not recently, so I'll defer to you about Shinzon's motives.
But, while we're talking about it, wouldn't it be more in keeping with Romulan philosophy to wait for the Federation to up the ante to a certain level before going balls-to-the-wall after Earth?
It would make more sense, from a Romulan point of view, for the Federation to fear them than to be destroyed. The resources used to destroy the Federation in an all-out war would bankrupt the Romulans militarily by the end of it. The Federation would be destroyed but at what cost to the Romulans?
Isn't "mutually assured destruction" what's kept the Federation, Klingon, and Romulan situation the way it's been throughout Star Trek? Shifting alliances aside.