Ultimately, I got the impression that Spock's plan was not specifically laid out. It was just an improvised reaching out to a people, opening the line of communication, in the hope that one day Romulans & Vulcans could see one another as kin again
On a personal level though it appeared he suspected a growth of potential within the newer generations of Romulans which could have been developing toward a more Vulcan way of being. This is what led him to believe in pursuing relations
I think he believes Romulans will someday choose to live like Vulcans, & has decided to plant whatever seeds of encouragement he can. He's not trying to convert them, but simply help them along & let them know they have a race of brethren that is there for them
His goal is logical, but his plan could not be. It had to be more intuitive than logical, because he was hoping to build a bridge with an emotional people. What makes it a great story for him is that he is taking all he has learned about coexisting with emotional beings, from his life in Starfleet & using it to bridge the gap to his people's lost kin. It means his service in Starfleet had a nobler purpose than just to serve. It gave him the wisdom to know how to go about reuniting two very different peoples. Spock may be the only Vulcan ever to have the wisdom & will to do that
One can begin to reshape the landscape with a single flower
The reason for my coming here has never been more clear. The union of the Vulcan and the Romulan people will not be achieved by politics or by diplomacy. But it will be achieved. The answer has been here before us all along. An inexorable evolution toward a Vulcan philosophy has already begun. Like the first Vulcans, these people are struggling toward a new enlightenment. And it may take decades or even centuries for them to reach it; but they will reach it. And I must help.