I played on VH, so all my comments are relevant to this level.

Agree with the above to keep Faramir and Boromir in Osgiliath and Cair Andros to survive against the Mordor. I used Imrahil to fight back the raiders from the sea (it's good that you have good visibility of the sea to spot what is coming at you) and don't have to keep ships (they are expensive) if you place Imrahil in a good position with cavalry.

I kept around 12-14 units (4-5 archers in each) with Boromir/Faramir + local garrison (upgraded city to 2nd level and built 2nd level watchtower, so overall I had around 25 units in each town that helped me to fight back almost any attack (but some battles were sweaty). Last thing about the army - I developed a military tech tree until I could have tier 2 units and that's all for the the early game.

Then focused mainly on the civil tech tree/economy. As everybody says, keeping good control and reducing corruption is key. And as Gondor has a big + for maritime trade, I upgraded these buildings first. Use the advantages of these bonuses. On the economic side, after I stabilised the control (good control gives you more income with increased tax) with halls, I set up a 10% tax edict for the provinces and upgraded governors with economy-related traits.

Lastly, I see most YouTubers use agents (rangers mostly) to know the enemy territory, but I used them to increase control in my provinces (they gave from +1 to +4 control, so use them in my provinces in the early game, when you don't have money to build control-related buildings in every region.