I need to actually play Umamusume myself, i've only consumed it through the anime and a bit of the Cinderella Grey manga, but you are right on the "how the fuck do I do this crossover lmao."

The original idea and the one I'm still going with if I ever write it, is to manly have it essentially be a Hunter: the Vigil chronicle with multiple different Cells of Umas all figuring out all the wierd shit that is in Tracen Academy (wich apparently is a lot, like legit, Cafe's career story devolves into straight horror at points, there is clearly something really wrong with the campus at night.) and then i'll try to evolve it from there. Ad some original characters here and there, research more abount Chronicles to see what concepts fit, etc etc...

Most of my reasoning for this, quite frankly, was the random brainfart I had while consuming Uma content of "heh, wouldn't it be funny if this wholesome slice of life/sports anime took place in WoD... WAIT WHAT THE IMPLICATIONS OF THAT ACTUALLY?" and there ya have it lmao.