try a fulmer snaffle for additional stearing aid. Preferably with a french link to reduce the severity found with a single joint. Then add a puller strap which fastens to the front of the caveson and to the bit rings. This transfers some of the pressure from the mouth itself and encourages the...
If it is ony tounge relief you are after why not go for a simple half moon snaffle. Either with eggbut cheeks or loose ring if you need more play. Unlike a jointed bit this is mild and moth horses like it.