Would be nice if you could limit factories to a certain amount of metal per turn, instead of them eating all they can. That way you could build ships sustainably, leaving a reserve of metal for insta-buys.

If you run out of metal, you can always just switch production to another item. Progress on previous items is always saved.

Late-game, at least, I don't really have anything to build but ships, and I don't usually use factories to build improvements anyhow (that's what money is for :) ).