The cost of outpost ships is certainly an issue. For one thing, they are effectively free once you reach a critical mass of outposts, which only further accelerates your ability to expand. Increasing the purchase price might work, but I would consider also eliminating the 25k CO they apply towards the outpost they create. I would also increase their build time.

I like your other ideas too, but unless they make some changes to the power of colonies, I'm not keen on associating any of this with orbitals, since orbitals are effectively untouchable early in the game.