I know this is a huge topic so just looking for some thoughts and shared experiences, as I've just started thinking about this.
DD (9) is a very gentle, quiet rider - good seat, soft hands. Her weakest area is effectiveness! We loan a 12.2 Welsh pony (DP), in his twenties. We solely hack together, no schooling, competing or jumping. We've had DP for 2 years. He has always been heavy in the hands, and inclined to lean. DD does a good job of keeping him active but I feel his eggbutt jointed snaffle may not be optimal for helping her.
There is no issue with brakes, but steering and control could be more refined! He just seems a bit dead in the mouth at times. For example, today he decided he wanted to walk on the (holey) verge, not the road. It then becomes an undignified battle for DD to get him off. He's an old boy, schooling is not an option. DD has weekly lessons and is a nice little rider, but certainly not good enough for any elaborate biting.
What are people's thoughts about other types of bit? Thanks for any help.