Good points.

Three simple ideas for consideration:

1) Make Outpost Ships more expensive.
2) Create an orbital like Shipyards that limits planets you can control. Unlike Shipyards, don't cap it at 10 of these (only limited would be 1 per colony). For every one of these new orbital types, allow x number of planets to be managed, whether they are colonies or outposts.
3) Allow x number of mining outposts to be managed per colony.

As someone who wants to play for 5-10 minutes per session, three times her day, I too want to cripple the land grabbing strategy I put forth.