This might be a long shot, and I'm not sure if it's been mentioned before, but here goes:

Using statistical calculations, bast could implement a population growth rate that is independent of immigration or tourism. This rate of growth would be extremely slow and would speed up in relation to the amount of colonists that you have living in your colony, while still being pretty slow even for a large colony.

The colonist would start out with zero iq and would not need a place to live or a job until they reach a certain iq. When they are spawned, they would start to receive a periodical supply of money from the two parent colonists that spawned them and would use that to buy food and water. Bast could also draw them as tiny colonist sprite. The priority for them would be to seek out constant supply of education, but would also need entertainment as well.

The reason why I say that they don't need a home would be because it is assumed that they would be living with one of their parent colonists.

Also, when the planet is fully terraformed, bast could put further purpose toward the family houses that one can build. Two colonists living in one of these homes would have a very good chance of spawning another colonist.

The reason why I wanted to recommend this idea was that just in case you can't pay your charter tax, you can still grow as an isolated colony, albeit very slowly. I mean, in real life, that's what would happen if a colony were to get isolated, eventually people would be born and the population would increase, but over the course of years.

Let me know what you think.