While I read with interest your Arthur comparison to Nemesis (and actually agree with some of your line of reasoning), this...
The plan was always to decimate Earth, but for Shinzon it became a pressing priority. If he succeed, then Picard, and everything he stood for would be figuratively null and void. There would be no Federation or history record of Picard's deeds of beliefs.
...is still kinda flimsy. The fact of the matter is, going just by what we see in the film, Shinzon's decision to attack Earth came literally out of nowhere. With all due respect to what you and Timo think, the movie gives zero indication that Shinzon had any intentions at all about Earth until Donatra and her ilk pushed him into finally attacking "the Federation." The only "pressing priority" on Shinzon's mind was fucking around with Troi and trying to get Picard's blood in order to survive (and both of which he did extremely ineptly).
If Shinzon really wanted the best of both worlds, he would have tried to attack Romulus instead. That way he could have taken out his revenge on his true
oppressors, and proven to Picard that he was the badass warrior he always wanted to show Picard he was. All destroying Earth would have done was show Shinzon as a sadistic butcher who killed completely innocent people.
Earth had nothing to do with this movie at all. The only reason why Shinzon decides to attack it instead of Romulus was because the writers probably felt attacking Earth would have had more of a dramatic impact on the audience than Romulus would have (which IMHO is a load of crap).