If you want a great short term fix, remove gamma, reset. Then simply have gamma's, with main termails with your said restrictions without terraforming available.
Your looking for a short term solution, and you dont have the resources for a long term one. Have the buildings make there own pad, without any need for terraforming. As well as, implement the CPU, and Power restrictions that are needed to fix the sprawl.
I would also suggest that if you are looking for smaller bases with smaller footprints, then you need to boost the effectiveness of all the defenses. I would also suggest that you reinstate OP being no closer then 2500meters from one another, and reduce how many can be on one island at one time.
Unfortunately I believe your looking for a dynamic Gamma scene with full loot drop, and you are expecting the players to do this with no Gamma incentive at all. This is a entirely different conversation however.
If you are looking seriously at inproving then game, and removing gamma castles, great, also think about re-adding Epi back into the mix for a better incentive to live out there, again.
The Gifter
Top Killer 2013 - 01: 334 -- 17 -- 317 : Merkle
Top Killer 2012 - 01: 027 -- 472 -- 445 : Merkle
Scarab Kill Count - 13