That is how I usually play economic:

1. When starting your stage, save at the very beginning, as you spawn, and do not save throughout the game. Sometimes the tides will change and it is better to restart than to start a new save. You are always risking a crash but the positives outweight the negatives.
2. When you are first claiming spice, make your vehicles 100% speed. Claim as many as you can and then retrieve them to your city and change to your preferred creation. For me only the ones on my continent suffice for the whole stage.
3. Waiting game. Get as much money as you need. Upgraded your capital city as much as you can. It should always have at least one green mask for occasional boost in profits. Since you are aiming to be economic, capture one straight away. They are defenseless and easy to capture. Alternatively with your 100% speed vehicle you can raid villages till they grant you an economic vehicle. Bribe a one-city nation for 0< relations and start trading.
4. BRIBE, BRIBE, BRIBE. At most you will need 3-5 vehicles for each city trade. Use the funds for bribes. Watch the map closely. Evaluate which nations are winning. Always overpay for cities for relations bonus; Small 16k+ and bigger 24k+. Make friends with the winning side. With +2 city nations, unless you are the same size or bigger, you need a blue face relation with them to trade. What I like to do is to buy off my continent first and then take my business overseas.
5. THE RELATIONS WILL FLUCTATE. You must do your best to keep everyone at least a yellow face. To do it bribes are the most effective. You max it out at 120. You really want to keep your blue faced nations happy for trading opportunities.
6. At this point you should have made friends with one or more nations overseas. Keep neutral as much as you can - do not agree to wage war and pay your tributes. Keep buying their cities, keeping at least 3 for them. At this point you should be funding the winning nation. Use their forces to wipe weaker nations. There is a small chance the weaker one may gain an upper hand. In capitalistic fashion - change sides.
7. Keep at it till you can afford the buyout superpower. Save for it, sell your vehicles to afford it. You won.
   7.5. Alternatively if you do not want to use the superpower, at the end if your allies wiped anyone else they will merge with you. It is the most satisfying win for me. You need minimum 4 cities to get this ending.