What if planets had varied amount of mexes you can build... mex on to increase that yield up to 4 metal per planet?
Mexes in game provide 1 metal income regardless of energy (no overdrive) and can be easily (say, factory's amount of hp?) destroyed.
Can be destroyed even without battle with bombers (some chance per bomber per mex to sucessfully destroy it). Costs 10 metal to build (will pay off in 10 turns).