Gymnastics, grids with poles before and after jumps. Get the horse thinking. I agree with canter poles set as a 5 stride. Do the poles in 5 strides, shorten strides to 6 strides, optional to do 4 strides but I wouldn't recommend that with a horse who is already rushing jumps.
My horse used to go galloping round courses and on the flat and didn’t really have a canter transition it was walk ,trot , gallop and now he has a lovely canter most of the time . Wat really helped was poles working on getting a distance between them for example one time get 5 strides the next get 6 then 7 and add loads of circles before and after and this will really get him thinking about what u r asking him for .poles are the best as he really has to think where his feet r going and when practising jumping put a few poles before the jump so he slows it up to think about wat he’s doing . Hope this helped 😁