Well the main reason I have so much metal is that I have so little of everything else.

No real candy income means that I rarely need to use that last big 12,800 and 25,600 stat increase, in fact I have only done it once and are about to do it again soon. The main reason is that I have levels all my important 4-5* girls that i'm using to affecting level 2 so 30 x 15 = 450 candy for that instead of 3 girls to max lvl 5. The reason is that it was cheaper on metal and candy for the rewards.

My gold goes to the factory and emblems so I don't have enough to repair, upping my emblems has been the only way I could proceed higher right now, the XP grind is quite hard in this game.

Metal is that one resource that I have little use for. The small amount used for factory is nothing really and repairs cost too much gold to bother doing, also PoC not needing repairs means that the best way to farm XP right now is PoC as high as you can go then back to easy missions to complete dailies. Based on that I only need to use metal once a week when PoC resets and I get some decent candy, since I get 1500 each major gather and a fair amount from 15 hr scouting runs, it builds up fast.