At what point should I turn in my CAC at DEERS?

Keep it until it expires. I assume your ETS date is correctly listed on your card. Don't dispose of it before or even when you go on leave because on the rare instance where your leave is cancelled, changed or some other reason you have to go onto a base you will need that card. You won't be breaking any rules doing this but if you self dispose of it when you are on leave and you have to come in you will have some issues to navigate. There are card issue facilities everywhere and are extremely easy to find on the rapids website. Just turn in your card when it expires.

That should be the last step.

I just dealt with this last week. One of my Soldiers was getting out. On his last day he came to the unit and showed us his new transition ID card. It gives him access to some benefits after leaving the Army. We then took his card. All IDs are government property. I wouldn't have called him back if he left with it but we asked for it before he left.