I think Shinzon made a lousy substitute for Khan...
Shinzon isn't trying to be Khan. Nero is the "lousy substitute for Khan".
Khan hated Kirk for marooning him on Ceti Alpha 5
Nero hated Spock for saying he would save Romulus and failing.
Khan blamed Kirk for the death of his wife.
Nero blamed Spock for the death of his wife.
Khan stole a machine that could be perverted into a weapon.
Nero stole a ship carrying a machine that he perverted into weapon.
Khan wanted to make Kirk suffer
Nero wanted to and did make Spock suffer
Khan was completely obsessed with killing Kirk
Nero was completely obsessed with killing Spock
Shinzon never focused on Picard like a laser like Khan and Nero did to their respective enemies. Shinzon priority was getting the transfusion and ridding himself of his clone material. For a clone it makes sense, because Shinzon longs to be "authentic", but can't while his original self is still out there living.
One of NEM’s myriad flaws is that it seems to forget Picard’s sobbing in GEN that “There will be no more Picards.” Shouldn’t there have been signs of hope, love, relief, something positive on his discovery that there actually is another Picard? It would have been a satisfying way to wrap up the TNG saga if a redeemed enemy became the vehicle by which the Picard family got new life.
Picard did try to embrace Shinzon, several times. In the Romulan senate, on the Scimitar while he was restrained but Shinzon's doctors, and in Picards ready room.
it's kind of hard to embrace the long lost son, when they show up in a dreagnought ship that has 3x the weapons the Enterprise-E had, having dinner in the place where your clone committed mass murder, and later discovering a biogenics weapon on his ship. At the dinner Picard was reluctant but was visibly warming to the idea of trusting his clone. When Geordie told Picard about the thalaron Picard had decided he couldn't trust Shinzon. Several minutes Picard was beamed off the Enterprise, and Shinzon tried to kill him.
Nemesis was always to be a hit and miss film. Paramount wanted Insurrection to be the last one, but Berman talked them into making a 4th. Had it been successful there would've been a proper send off to the TNG crew in a 5th film.
It is my personal opinion that VOY and ENT used alot of the story and monetary capital the TNG films could've used to keep the films going.