have 5 colonies 1 have an income of 600k/m 4 others ~250k/m each, around 1.5m per month, yet at some point, you don't have on what spend the money anyway. Plus you can get any number of supplies in your colonies which is critical.
Omg thats insane. May I ask how?
Spoiler
(https://i.imgur.com/ztJeXDw.png)
First - accessibility, planets don't even have to have good resources or low hazard rating. Second - starting budget, be ready that at first, they will not bring you some significant amounts of money instead you will pump into them a lot. Third - do not build your first colony far from core worlds, the best is to scan those 3 free core worlds systems and pick the best one, settle there.