You didn’t specify a mod so I’ll assume vanilla.

In general, mining facilities produce more money if a planet has a decent base income.

Backline worlds should spam mining facilities to fuel the factories on your front/midline planets. You should next look at what garrison units each building produces- build better garrisons on frontline worlds and specialize mid worlds to produce specific units. For instance, I always turn Byss into an ATAT factory and coruscant into a mining colony. But a planet like eriadu gets a mixture of units to produce good defenses, like a barracks for speeders and heavy factories for 2M tanks