why would not colonize a 30 planet unless its close to another players territory. Then you would have to conquer it later. you don't colonize bad ones like volcano. You get lots of productivity from a 30 planet.
Did not say colonize it just not core it, Way too many 10+ planets way to close to each other weaken Core worlds using some as colonies especially if you have 2 or 3 "large" planets in the same system make the Cores stronger. It is a judgement call and knowing what resource is more important than another helps.
Like if you need the resource waffles for both food and approval, but pancakes only gives food then you would Core the waffle planet and colonize the other then when you get the tech that gives orbital harvesting you can use that to harvest pancakes.
It's a learned thing but you should get the hang of it.
Plus you can always go back latter and put a Governor on the colony.
