On what should I spend coins?

I got so many free dice rolls, and consequently so many premiums-free dice rolls that I got a lot of coins left after building to lvl50. I don't want to waste the 6M when rebuilding, but I also don't have enought for tech tree upgrades or landmarks (Factory Landmark on lvl4/7 so completing it is 22.5M). Should I wait to see if I earn enough? Should I forget it and rebuild already?