$o… Many… Failed experiment$$$$ (This is not an uncommon theme with the breed).
First off, he’s a curb chain kind of dude and as primarily a cde pony I long ago gave up fighting that reality, but most of these mouthpiece options are what I used in a loose/direct ring:
His dressage bridle is a Neue Schule verbindend and the warmblood Weymouth, which is just a Conrad style (big square port, 45 degree forward tilt, great for that big old tongue).
On marathon/cones he uses a trust medium port, which is really just a forward arch. BUT I’m using a direct rein so there’s little to no curb action or bit rotation. Or collection for that matter. 
But that’s ok, the point is to have zero delay in turning aids which is why you avoid any leverage
In driven dressage we played around with a Conrad, pivot and it was good but he was still leaning which meant the curb was in play a lot (and if you loosened it, he leaned more which is how I started to figure out it was the rotation that was the issue), but then we tried an arch. That was good, but the dude loves his pivot so we found a pivot arch. That’s been the best so far because what I’ve figured while he NEEDS the big space for his tongue and the conrad fits the bill, he’s not thrilled with a Conrad rotating in his mouth, so that bit works best with a snug curb chain. That’s not a huge problem with the full bridle because I can ride off the snaffle and just periodically reinforce with the curb, so that snug curb chain isn’t an issue. But driving aids are primarily off the bit and there is no snaffle relief, so that was harder on him. And you would think that would mean I could just use the verbindend since I ride mostly off it and he collected and light on it… #butyouwouldbewrong One of the worst tests I’ve ever had. HE HATED IT and did everything except fling himself in the dirt to show his displeasure. Does that make an ounce of sense? Only if you have ponies. He’s really not subtle in his response to bits, good or bad. But he goes around a 3* driven dressage test with a fairly loose curb chain with the pivot arch because arch bits just don’t rotate like Conrads and apparently it’s enough tongue relief for him (he would let me know if it wasn’t). At some point I thought I’d look into that as a Weymouth option but I’ve not seen a pivot version, it’s most likely not ridden dressage legal and he’s really happy in the current configuration, so even if it is legal, I’m not sure I’m in the mood for yet another $300+ experiment.
Bits I’ve tried and he really hated: every 2 and 3 piece snaffle ever made, Waterford, happy mouth mullen mouth and assorted myler bits in my box. He sort of liked the bomber tongue relief which led to me trying the Conrad, but the challenge I’ve had with virtually every conrad/tongue relief style out there is they have a one size fits all port. He’s almost a 6" mouth and he has a bigger tongue than a horse with a fat tongue in a 5.25 mouth, but damned if bit makers realize that. Most bits seem to have a small port except for neue schule (it’s at max width and height). So far the trust medium port and the innosense medium port work best on those days I torture myself with a snaffle. Even though they have a forward arch that’s not that significant of an effect in a loose ring.
Probably way more than you wanted to know!
