Re: metal layers
if you use something like 11 layers of metal, usually the top level metal is aluminum not copper for reliability purpose in order to connect to solder bump(C4).
And if you design high performance dense chip, the capacitance between wires at same metal level is far larger than the one between different metal level.
And the metal thickness is not something you can control, you got what foundry provided, which basically define your resistance per box.
You need to think about performance, power, electromigration and routing area together.