If your mares are registered Holsteiner, even Australian Holsteiner, why not consult with your registry or the German Verband about their thoughts on Bonaparte N,? I know Celle is the Hanoverian Verband, but many of the horses they stand are also approved Holsteiner–as is Bonaparte. The Holsteiner Verband has Waterdance xx, Armand xx, and Cyrkon xx, but their blurbs say nothing about whether they are available frozen for export.
I know you’re breeding for dressage, but the Holsteiner Verband owns an SF stallion named Fidelio du Donjon who is 96 % TB, very tall and rangy, with a pedigree to die for. He was a GP jumper himself in France. He takes a very different ride to excel than the German style, but that may just be the difference in training and riding style.
Private Holsteiner stallions that look very interesting are the Heraldik son Herald and Mighty Magic who has Mytens for sire and Heraldik for damsire, so he’s 3/4 TB. Although MM has been bred for eventing, his dressage number is actually higher than his jumping number. 128 (85%) versus 108 (75%). The Olympic dressage horse for Poland Ekwador (H-Ekwador) WAS a stallion, may still be, and is by Heraldik. Mighty Magic has more than a few foals on the ground in the US, but most are out of TB mares. Refining wasn’t a priority. But the Verband or your registry in connection with the Verband might have an opinion on MM’s refining potential.
For a Trak who is long and lanky with a spectacular pedigree take a look at Laurel (Stan The Man xx out of a Pregel daughter) who stands at Marbach (Bad-Wu). All of the ones I’ve mentioned have exported frozen or will be exporting frozen, except possibly Herald, Ekwador and Fidelio.