Your horse needs retraining. Bigger bits is not the answer. Any horse can be ridden in a halter if your willing to put in the time and effort. People who don’t buy bigger bits and tie downs. It depends on how you want to train your horse.
Multiple ways you could approach this fin he’s strong, there are rubber bits, rope noseband hackamores- which is a good alternative which can turn a horse completely responsive compared to with bits. Maybe try a trust bit too? :) good luck