I don't think that timeline works though.
We saw Ushimaru in Yamatrash's flashback. That means he was still in Wano say 23ish years ago. Zoro is 21. For Ushimaru to be Zoro's grandfather, his daughter would have had to have fled Wano 21+ years ago, abandoning her father before Kaido took over, but somehow Koshiro and Kozaburo never treated Zoro as part of the family, even though he is technically Shimotsuki. It also basically torpedos the Zoro x Tashigi ship, given that she would now bear resemblance to Zoro's dead cousin.
I could be wrong, and Ushimaru may be related to Zoro, but Oda would have a lot of explaining to do.