As you are remnants of the Númenórean fleet and have no home and no liege, the commander and kinsman of Ar-Pharazôn, you have claimed the crown and call yourself the Emperor of Númenor. In the past years you have sent spies around the coasts of Middle-Earth trying to find a new home.
The ruins of Tharbad have caught your eye for a number of reasons. Having Númenórean infrastructure that can be repaired with ease is the important one. It being abandoned makes it the perfect new capitol. This is why that location has been buffed to grant increased free upkeep slots. Do not conquer it on turn one because if you do, the auto expansion of turn one will destroy your forces and potentially rout your faction. Start laying siege on it at turn two at the earliest.
Then you have two sons who are your commanders. They both have clear orders. Gimilzor's line will remain in the vicinity of Tharbad in order to reclaim Lond Daer, an ancient Númenórean port and limit the authority of that tool of Valar located in Isengard. Meanwhile Gimilthon's line is ordered to march north to claim the Elendili holdings surrounding Fornost and to destroy Sauron the Deceiver's forces in Angmar. If you want to claim the rubuilt Annúminas, you need to protect Aragorn until he gets it done. Hatred of Sauron and Valar are the greatest themes in an immersive playthrough of the Golden Dawn.
Where it comes to allies, dwarves are the number one choice of the Empire. They aren't loyal to Valar and dislike Sauron's minions and elves alike. The Elendili factions are secondary allies as they are still Númenórean even if their ancestor's were traitors to Ar-Pharazôn the Golden. However, these secondary alliances are always meant to be secondary. Haradrim can be an ally as they are descendants of King's Men but should not be trusted until Sauron has been annihilated. Elves can be used as tetriary allies but should be betrayed as soon as their presence becomes an inconvenience.
Hope this helps you in your glorious conquests. ^^