Bit problem(gear rules)

I am about to lease horse for eventing and show jumping but I’m having a bit of a problem, the man who rides him currently has him going in a Dutch Gag (which I can’t use for dressage). He is a very strong 17hh thoroughbred gelding who has a hard mouth and pulls. I am looking for suggestions on a slightly stronger bit that is legal for dressage (my preferred bit is an eggbut French snaffle but unfortunately it would to soft for him) and .

I’m more looking for suggestions on bit/noseband for more clarity to him as he needs the slightly stronger feel because of a near dead mouth(I’m assuming this came from being a steeplechaser then going on to hunt for many seasons.) He has done Grade 1 PC(AUS) eventing but I have no contacts to find out what he has been going in previously.

You’re not going to be able to find anything that’s both legal and at all strong. A Dr. Bristol is as strong as it gets (at least until you can use a double.) Experimenting with a drop/ figure 8 etc. can help. But it’s not really a bit problem, it’s a training problem-- and if you can’t control him in the dressage ring in a snaffle, I think you’re going to be in trouble on xc.