This is the best write up I’ve seen on the topic. Also interesting to learn that FINRA is not a US Gov’t agency. It’s a “Self Regulatory Organization” organized by stock brokers to make sure that things are running smoothly and the SEC and Treasury doesn’t start to regulate the stock brokers themselves. You can guess who SROs work for.
There’s no way to make your account secure. These ACATS transfers are executed upon receipt without much if any scrutiny by your broker.
I think you want to split your money between 2 different brokers and hope an ACATS theft doesn’t affect both at once. There doesn’t seem to be a problem that you’ll actually lose money. But you could lose access to the account for 2 or 3 weeks while they investigate the ACATS transfer theft.