I posted this elsewhere earlier, but it's my view that if things do get worse, they will do so across the board. The thing to look out for, then, would be which broker is in a better position to marshal enough resources to fixing the problem. TDA has money and personnel to throw at the problem, and they would do so eagerly just to prevent customers from jumping ship.
We know that TDA has had trouble, now and in the past. We also know they fixed the problem in relatively short order. We have no idea how the newer kids on the block will fare under similar stress.
So I would probably open an account with another broker as suggested, and keep it "wet" to get a feel for how they perform. Over time I would then increase funding a the new place, maybe at the expense of the incumbent broker, or not. (I did that with Tradier recently, and also have active but unused accounts at Tastyworks and ETrade.)