I use a KK ultr (double snaffle with a "bean" in the middle) the most of any bit I have. It is a loose ring and I absolutely LOVE it and have not found a horse yet that doesnt do just fine in it. It is not particularly thinner than any bit, tho. However, you can get them thinner.
I have a Myler comforat full cheek. It is ok, too.
In most cases, outside of a really hard,or sharp, or fat bit, any bit is ok if it is used well. In your case, you have a special situation.
I will be so curious to hear how your choices work out for you. It's really great that you are taking the time and effort to make this situation better for you horse.