While producing with workers will provide the bulk of most players' resources, trading has some benefits over producing that should not be underestimated. For example, trading for a desired resource is sometimes preferable to moving a worker to a territory that provides that resource. The action that you would have spent on movement can now be used for something more productive. Trading also allows players to access resources that aren't available in their homeland.
Just because an opponent has a lot more power than you doesn't necessarily mean they'll win in combat against you. They don't know how much power you will spend or the amount of power your combat cards will add (combat cards aren't worth anything at the end of the game, so spend them early and often). The puzzle of combat is outwitting your opponent, especially when they think they're going to win.
Don't underestimate the power of being able to move every turn. Without a Factory card, you can only move once every other turn, but once you get that card, you become much more nimble and responsive than players without that capability.
You can't place more than 6 stars, so it's generally better to focus on completing 6 specific goals than trying to do a little of everything.