Scratch the pedigree of the GRP and find Welsh and British Show Ponies with smaller sized WBs added more recently. All, of course, chosen and bred with German care and attention so an accessible performance history is generally available. That can make it easier to choose a future prospect. The WB is coming to dominate the pony blood.
Welsh sec D Cobs are common in the UK and so are popular for dressage and they do a fine job of it, even winning national titles. They are also usefully dual purpose as most can jump. They have good gaits, strong hocks, flashy looks and a lot of motivation. However, the thing with Welsh is that they have a brain and a strong sense of fairness. They really appreciate a sympathetic rider who is willing to work with them rather than a demanding rider who is bossy and dominant. This tends to mean people either love them or believe they are too hot. The smaller Welsh are less common in dressage competition because their riders are generally small children - but they do exist.
A rare breed in the USA is the New Forest Pony. It is the least “pony” type of all the British native breeds and a good one is more a “small horse” (max 14.2 in breed standard) with good paces, natural balance and a lot of scope. They have very good minds and, personally, I usually find them to be nice people. It might be worth looking to see if anyone is currently breeding in the US.
Connemara have become a fashionable pony breed in my life time. Once they were rare outside the west of Ireland, now they are world wide. Increasingly, the problem is finding a good one as breeders sell over-height mini-warmbloods at the expense of pony type and, perhaps, character and mind. That might not be an issue for dressage but it is a shame for the breed!
Any pony cross with TB will likely produce a useful horse, with blood for athleticism and pony for toughness and self preservation. It is noticeable how frugal and how little vet care Natives require compared to TB and WB so running costs are substantially lower.