I have a coming 5 yr old that really chews the bit. She goes best in a plain Dee snaffle but that's also what she chews the most in.
Some of the other bits I've tried:
Loose ring - does ok but still chews
Mylar comfort snaffle - chewing slowed but stiff jaw, head went up.
Mullen mouth - stopped chewing but stiff jaw, head went up, no more bending, dry mouth
My coming 4 year old is the same way. on a friend's advice, I recently switched him to a JP Korsteel dee ring with an oval piece in the middle. He seems to fuss with it much less. I think it lays better in his mouth and keeps him comfortable. He stays very soft, bends as well as a green bean with a gnat's attention span can, and no dry mouth.
~ A true friend knows all there is to know about you and still likes you. -E. Hubbard
We started using a bit at our barn for our young horses that (I believe) is called a KY Rotary bit. It moves every which way and must be very comfortable for the horses to have in their mouths because they all love it! There's one that was always flipping his tongue over the bit no matter what you put in, so he had been going in a hackamore, but with this KY bit he hasn't tried it once! Not sure if it's available in NA though?
I'd rather be riding!
Try different thickness' of the dee or loose ring (I'd try the dee first or maybe a 3 piece dee with the egg in the middle)? I find many people put really fat bits in horse mouths who are trying to find the gentle-est bit...some of which work well, but some it's so wide it bruises the bars and can cause discomfort...
Here's something else about bit fit. I recently had the dentist out. Told him what she was doing and he wanted to see her with the bridle on. He was like that bit is too high, see her smiling. Well, thing is, two different trainers raised her bit from where I had it. I use to ride in it where the dentist said it should sit. Didn't effect the chewing either way though.
I had the same problem with my then 4 year old last year. He went through 3 Happy Mouth snaffles, both D and full cheek variety; chewed enough of the mouthpeice that I threw them in the trash. He then did the same to the black rubber D so I tried him in the Myler D and he is much happier. His mouth was changing too, so I am sure that had something to do with it. I am very happy with Myler bits in general. I think they contour better to a horse's mouth conformation and lay on the tongue well. When used, they do not poke the roof of the mouth as I think other bits can do...
The stiff jaw/head goes up may also be more of a training issue, especially with the Mylar Comfort snaffle that actually has a bit of lift to it, almost like a very mild gag.
IME, the busier the mouth, the fewer pieces in the mouth. Mine all went better with mullins or another unjointed snaffle. Almost like they scared themselves with too much going on plus it tempts the nervous ones to play even more.
But they will not magically go better the first day you try it, especially if they are not well broke to the aids. Sometimes a new bit is best introduced with some lunging in side reins and week or two of very basic flatwork before you decide it does not work.
When opportunity knocks it's wearing overalls and looks like work.