> I believe geothermal has high capital cost and low running costs

Higher capital costs, but not nuclear high capital costs.

> That means it's competing against batteries, which are also great for smoothing out daily cycles, and are very inexpensive.

It likely would supplement batteries rather than compete against them. A battery buffer would allow a geothermal plant to slowly rise to load and fall as that load goes away.

A very large battery can store 200MWh worth of energy. The largest geothermal plant produces 1.5GW. (A lot of the large plants look like they are in the range of 100->200MW). Presumably those plants can run for more than a few hours which ultimately decreases the amount of batteries needed to smooth out the demand curve.