Seasonal storage is an interesting idea. Rare grid outages seems pretty easy with batteries if the South Australia example is anything to go by though, and for medium-term storage there's also pumped hydro -- not sure where that compares in cost?
On top of this grid operators want generation redundancy in case an individual power plant goes offline for whatever reason. Combining both you get quite a lot of excess capacity compared to the current grid.
You could subsidies that by using your batteries to energy to peak load times?
https://reneweconomy.com.au/south-australias-incredible-week...
South Australia aims to reach 100 per cent “net renewables” within a few years – over a full year – but in the past week it has already done better than that.
If there's "a major grid outage" no large scale battery (or gas fired plant or coal fired plant, etc) will prevent blackout .. as there is "a major grid outage" preventing power from being delivered.
Individual homes would be left to do what individual Australian homes do during bush fires, floods, tree falls, etc and turn to gas lanterns, home generators, and any community hubs that feed local rooftop solar through a local small scale battery (not common as yet, but they're about and increasingly so).