I use a myler d ring and my horse likes it so much better than a single jointed d ring.
Im currently on a trial run with my trainers bit ( herm sprenger dynamic RS arugian eggbutt style bit) from his korsteel cooper mouth roller d ring... my trainers bit is a bit larger and has a nice curve so it sits nicely in his mouth. He usually has the tendency to hang in his bit but has been a lot softer in my trainers!
What about a kimberwick? Idk if this is going to help. One of the school horses I rode was heavy on the bit and he was ridden in a simple snaffle bit. Turns out he hated the bit moveing around in his mouth. This bit is one solid piece of metal and is much less mobile
I would go for hard rubber and no plastic
What if you try a happy mouth bit?