Thank you for actually running the numbers. I think the data is quite convincing that overprovisioning won't be the solution to the seasonal storage problem, or at least not the major factor in it.

Personally, I have high hopes for flow batteries. Increasing storage capacity is so easy with them, liquids can easily be stored for a long time, and it would even make long-distance transport by ship feasible. If only we can find a cheap, suitable electrolyte.