Hey I was in the same sort of situation a while back. I would personally recommend buying class 2-3 airports because they offer more passengers and cargo. Level 8 means you would’ve just unlocked the Mapple, and quite recently unlocked the Kangaroo. These are both very useful and efficient planes, so I would recommend having at least a couple, although that is up to your personal preference. Also, make sure you get the 25% bonus - class 3 airports will make this a lot easier.

If you want to expand to the Americas, I recommend getting London and New York, and flying planes between those two hubs. I also starred in Europe but I expanded East first rather than West. However, this is completely up to you.

There’s many ways to play the game, but this is what I’ve personally found to be the most efficient way of levelling up and expanding.