Make it only be cast by Titan (similar to Jarrasul's Drain Planet), make Strip to the Core last 1 minute again to all planet types, and add it as Lvl 6 ability to the Titan, instead of The Maw. Make it a channeled ability and you gain all the resources at the end of the ability cast-time (like 4 minutes CD). Should fit better to them this way (also, make this ability usable only after the same Research chain is done).
Or, make the research chain as it was before, and have it 3 levels, with a Planet maximum to each level. Let's say 3 levels, and each allows a player to tuck in X planets of any type (like 2-3 per level, how many would you require after all?) Or the levels would allow planet types to be stripped (like 1st level Dwarf Planets and Asteroids, 2nd Vulcanic and Ice while 3rd Terran and Desert planets); again coupled probably with a maximum number allowable.
Should this suffice?