Batteries are not appropriate for dealing with Dunkelflauten. There's very little energy flowing through there, so what you want to do is trade lower round trip efficiency for lower capex. The high capex of batteries is best amortized over many charge/discharge cycles, for example for daily storage.

I mean, who cares? Fire up the gas plants in the one week a year you have weather anomalies. We’d still be 90+% carbon free which would be incredible. The last gap can be solved at a later point as technology evolves

It's not. Germany would need an insane amount, about 3twh based on recent data and much more looking at 30y weather data

Batteries can store as much energy as you are willing to buy.