It looks to me like you're on the right track as far as colonization goes. The game is intimately linked to population and economy. Colonization can have a huge negative impact on money.
Keep your current colonization strategy. It works.
The key is population growth. Try to keep your less populated planets at 100% morale as long as you can. This will devastate your near term economy, but it'll recover with your population.
Don't build any factories or labs on any planet other than your homeworld at first. When you start getting money from your other planets, start building structures on them as you can. Keep an eye on the economy and morale.
Adjust your taxes to grow your population on new planets, but to reap as much money as you can on your older ones. You'll find that staying on the extremes works better than looking for a balance. Keep adjusting your taxes per turn if needed.
Don't build too many farms. You might need one or two per planet, but more than that could kill your morale later.