Nice mare!
Something no one else has mentioned, but would you consider breeding to a Cleveland Bay (or even a CB/Tb cross)?
Anglo-Cleveland’s make great eventers, and are quite amateur friendly. (I know- I’ve owned them for close to 30 years now [my screen name was my Cb/Tb Preliminary eventer +, and my avatar is her daughter by the real-life version of a much- respected member of this actual forum [Fred] (so 3/4 Tb-1/4CB)]).
I don’t have any specific stallion to recommend, but most of them meet the standards you’re trying to improve- they’re known for good, strong hooves and bone, and are usually fairly short coupled, not that your mare is long backed at all.
Good luck with your mare, and whatever choice you make I hope everything goes well and smoothly for you and your lovely mare.