The best storage solution is to offset normal hydro generation to build up capacity to be released when you have unmet demand. That massively changes the need for storage because dams are already storing months worth of energy so shifting demand within the day is effectively free barring possibly adding some turbines.
Globally 16% of electricity is produced by traditional hydro annually that can cover the majority of the projected need for storage in a pure wind/solar grid.
Also, by the time we need significant batteries the costs will have fallen even further. If you want to eventually cover 10% of the grids daily demand from batteries using projected costs from 2030 to 2040+ it doesn’t look unreasonable.