If you’re really enjoying yourself I’d considering picking up Phantom Liberty as well. I played through the main game shortly after it came out on PS5. I was very ‘meh’ about it. But I recently picked up Phantom Liberty and started a whole new game, I probably doubled my play time, and still spent 4-5 hours after I’d finished all the story missions, just bumming around bc I was still so enamored of it. The story missions in Phantom Liberty are, in my opinion, better, bigger, and more emotional than the main game.
I’ve heard people say you should start a new game. Which I did. But I think you’d be ok to jump in if you wanted. There’s a new neighborhood that it mostly takes place in. To start it, you have to complete some main game story mission about 3/4 of the way in, if I remember correctly.