I like your idea of the loose ring Myler barrel snaffle. They aren't very thick, they have the loose ring action (I love loose ring because your hand to bit orientation stays more accurate no matter where thehorse sets their head and it disallows them to lean on it as easily as D bits, though I also use D bits just fine, too)
I think you will like that Myler bit! Be sure it is wide enough, and when in doubt, err on the side of a tiny bit too wide as opposed to the opposite. Sorry if I am telling you something you already know.