TreasuryDirect is about as safe as you can get and you can roll everything over in short term Ts.
I would not open a securities account on the assumption of hiding your money there. Other than treasuries if the banks runs hit hard most investments will likely dive and you won’t be getting all of your capital back insurance or not.
Careful with Credit Unions as well. They run under NCUA insurance but there is a lot less in there to cover failed banks than FDIC. I bank at Navy Fed and they were selling ARMs and 100% financing all over the place. I don’t believe they will be hit but if they were they would likely drain the entire NCUA indurance pool themselves.
