Hey,
I love your mod but I have one idea/suggestion. In my opinion the option to make the tanks lightwight is kind of op, because you you always could get lots more Delta V by just pressing a button.
Of course it's just useful in the first stage and in a upperstage with fuel which doesn't boil off. To bring realism the tank could be made out of carbon fiber? Than the fuel tank would be more expensive (20% more?)
Thank you!