Some of these animals need to learn “to go into the bit” rather than try for the “lightest” bit possible. Does not mean he gets heavy mouthed, hanging on you, non-responsive to reins. The problems arise when horse over reacts to a little rein pressure, won’t go FORWARD because he feels some rein weight or pressure. No forward is dangerous!! They usually will add backing up when they won’t go forward, won’t do Whoa either! You may hate using the whip, but without a ground person to grab pony, that is about the only alternative to getting forward again!
Equines do notice small changes in harness, different vehicles. Our tell us the leather reins are TOO HEAVY after using the Web reins! Heads down by their knees for a while, then at their normal head carriage when we start trotting. Too hard to trot with heads that low! After a day or two, they forget the web reins, leather weight is now the normal feel.
Driving reins are longer than riding reins, have more leverage with that length over riding rein weight. Pony needs to get used to that. As a Driver, you can work on being subtle with the reins, wrist roll instead of an actual pull. Take a small loop for corners, instead of moving your arm back.
The pony may need to do some " homework" too. We do the old-fashioned mouthing with our young horses, newly purchased horses, so they are familiar with a bit in their mouth, no big deal. We start with them wearing it in the stall, then during any grain feedings. They wear it for an hour to a couple hours daily. I want them to finish by just holding it while relaxed in the stall or tied to a post. They (usually) will quit chomping bit, their mouths get tired, like with old gum! Bridle and bit become non-exciting, not any reason to react. We did and do, have have a mare and son who are mouthy, busy lips. They both used a loosejaw bit where they can raise and lower the mouthpiece while standing still. They just needed that “fidget outlet” to be able to stand quietly in harness for very long times. Her other sons did not need a fidget toy, stand well. Husband says “” Better an outlet you can tolerate, over them going to real misbehaving! You just ignore the tink-tink of the bit jingling