When I had a bunch of cash from a house sale I first broke it up among various online banks that paid higher interest and kept amounts under $250K. Then as treasuries/rates have gone up, I moved most of it to Schwab and invested mostly in various treasuries and a small amount (10%?) in the market.

The insurance for brokerages seem more confusing to me. SIPC limits but most all brokers also have other insurance. The catch with the "extra" insurance is that it has an aggregate limit so if something terrible happened with a brokerage I would expect that limit to be hit quickly.

I do believe not to put all of my money in any one brokerage, bank, credit card, etc. I've had cases myself and seen others temporarily have money access issues which I'd prefer not to have to worry about.

I think if things got really ugly I'm not sure how you could hide from it. If it hit banks hard then it would filter down to all kinds of businesses that need access to credit lines and their own money and would just cripple the economy. Always can buy physical gold but I'm not sure how easily you could use that in a catastrophe.