My idea is that most Hydro-Power plants can be used as reservoirs with little modification.

And biomass can be burned at whatever moment you like (provided you have the plants to do so), so it is “on-demand power” in some sense.

Instead of recycling rare metals from batteries (which is notoriously difficult), try constructing your batteries without rare metals. It is theoretically possible, sodium ion batteries set an example for this. Yet they are still under development. Link