There is no law of diminishing returns. That’s an aphorism, not an actual scientific principle. If your power source can generate the power, it does so.

You don’t need to store an entire county’s power per day. Thats never been anyone’s goal, nor is it needed. You generate power for at least half of it, then continue to generate power with other green sources while also storing it.

You need to “restudy” the current state of battery tech and geothermal. There are huge arrays of different batteries being built now, and active 100+MW geothermal plants in operation, with more coming. The tech is here now, being built as we speak.