Prefab bases seem unnecessary to me. IMO building structures in orbit (at significantly higher cost!) and dropping them to the planet's surface seems like a thematically consistent way of approaching orbital invasion. I suspect these constructions should be immobile, and be vulnerable while in orbit.
Well, if you invaded properly, you secured a portion of orbital. If you cant secure one, they had 10+ minutes to fortify planet, you lost your chance, work on building halleys. If you can, build your own anchors and SXX. Actually, you can choose location of orbital landing, you can theoretically just land SXX overhead of umbrellas and they shoot first. Anchors, you could technically swarm 1 with 100 avengers I think if they were all to land point blank on the anchor.
You know what we need? A single use multi-unit transport that crashes into the planet, doing some damage to whatever it crashes into and dropping off a handful of units.
I've seen this idea a few times in the Backers Lounge, and it looks pretty solid. Some of the ideas thrown around were "D-Day style" planetary invasions, with dozens of these one-use ships throwing themselves at a base to form a beach-head, then clearing a path for teleporters to bring through the big guns. It lets units play more of a role in the interplanetary theater, rather than the current interplanetary nuking strategy that essentially invalidates (or at least vastly outclasses) any other forms of warfare save planet-smashing.