Agree with training but safety is important. I go with a double jointed Tom Thumb Pelham, two reins. If that's not working, metal ported Mullen. If you want a smaffle my go to bit is a circular slow twist, not a square slow twist. It's softer. A Waterford is also very good for horses who are strong.
I rode an ex high level eventing morgan once who had the hardest mouth at 24. Too late to train him to be softer. Tried a snaffle and I almost broke my arm being bolted with and coming off. I rode him in double reins, and a Mullen mouth pelham. Worked wonderful.
training. a strong bit will only cover the problem for a short time, and then the horse will have an even stronger mouth.
Try a rubber Pelham with a soft curd chain
But for more sensitive horse who is too strong on Mullen maybe waterford as its hard to lean on so many stronger horses goes nicely in one or double broken. But yeah all gag type bits like could work too
Those might work nicely 👍 But the Mullen mouth can be really good or really bad, as many strong horses will learn to lean and pull on it or lock on left or right side. Just a thing to keep in mind. Specially with rubber as it's usually thicker than normal mouthpiece and horses like to chew on it quite much