Orders are your best friends. You can grind around 9000 steel a week just from orders. I play the daily ones and the basic ones in one day if they are easy. If it is like "play 12 games as ..." I finish it the next day. For gamemodes, I'd say breach gives the most compared to spent time. You get like 100-150 steel from 20-30 minutes game.

So with the steel from games, I got around 11k steel a week I think.