Well, Starships are a good way to use up a bunch of excess resources -- but they cost a lot of knowledge, too. If you are saving your knowledge for future Mark II/III unlocks, then you might want to just hang onto that K, then. If that's the case, to prevent your excess M/C from being wasted, a Mercenary Space Dock would be the way to go, with some mercenaries cranking out. In the expansion, Golems will be another major way to spend a ton of M/C if you have a buffer (when you find a Golem, that is).
Though, whenever I hear about someone having so many excess resources, I immediately jump to one of the following:
A. You aren't attacking enough, and incurring enough losses in the process.
B. You are playing on a difficulty that is below your actual skill level.
C. Both of the above.
If that's how you want to play, then that's cool -- I'm not here to tell people how to play, and we have a wide variety of playstyles here. But when you play a very turtlish conservative game (A), or when you play on a level that is too easy for you, you'll tend to amass tons of resources where normally players are hurting for that income (or have a barely-comfortable buffer at best).
Mercenary Space Docks were specifically added to the game to combat resource waste in just these sorts of situations, when building more Starships is not an option. They are basically Mark II fighters/bombers/cruisiers, but without any ship cap whatsoever and at 10x their normal cost, so you can use an overflow to really crank out more ships if you are inclined.
