First, here are the flight routes I had in the end



In the early game keep routes as short as possible and use only level 1 planes. Add second level 1 plane on Cockburn Town - San Juan route whenever you feel comfortable with other routes and then same thing with George Town - Cockburn Town route. Start adding level 1 planes on other routes when cities are beyond yellow state or upgrade cities.



At this point I had reached furthest I had ever been and saved the game. All planes are still level 1 and most of the cities are level 1. Only Santo Domingo is level 3 and San Juan and Port-au-Prince are level 2



Here is where you start upgrading your planes to level 2. I started with all routes from San Juan and George Town. Save money until you reach around 1400 and unlock Aruba, this is a long route so start with level 3 plane.

After unlocking Aruba the scenario didn't feel hard anymore and I don't even remember been close to loosing. In the end I ended up opening an alternate route to Cuba to ease the traffic and also Basseterre - Roseau route to ease traffic in Venezuela, but other than that I felt the hardest part was over.

Guadeloupe is the final country to unlock and it costs 23737. I had to sell all my routes to unlock it







Hopefully this helps with the struggle.