The Myler Combo is, mechanically speaking, a piece of garbage. It sends conflicting signals. It is a gag bit unless you use the ring that the mouthpiece is on. Gags tell a horse to raise its head. The leverage applies poll pressure, telling the horse to lower it’s head, and the noseband squeezes. That’s a lot of conflicting cues, and while it may give you the whoa you need, it can cause other issues. You would be better off finding a decent snaffle, Myler or otherwise, and spend the time to train. If you feel you absolutely need a bit more control, try a Kimberwick or use an Argentine with two reins, one on the snaffle ring and one on the curb. Then it’s there only when you need it.
The combo bit, or any bit, is going to be a bandaid if his brain is gone after a spook. I suggest finding a trainer that can give you some techniques on how to deal with the issue. I totally understand not wanting to be left high and dry with no control, so I see why bitless may not be your best option right now, but I feel like the Myler Combo is only going to help you for so long, if it does at all.