I'm a shitter in need of guidance: my Uma sucks and I want to get better stam/pow parents.
Vaguely tried to understand how it work from guides, but unsure about how to pick grandparents.

Roster is pic related.
From what I understand, actual parents should have an inheritable Unique, so here that mean building Teio/Oguri/Taiki (+Agnes/Makenifucka for Christmas Oguri, apparently?) - which is already a problem since Taiki doesn't run the same fat loop of Medium races than the others... do I just *force* her too, by upgrading her "E" Medium?
Children will be mostly Oguri/Goldshi/Agnes (or whatever other random horse I have doesn't suck for the CM of the week), so I shouldn't use them as grandparents, only as parents.
So who the fuck do I use as grand-parent. Just whoever can run the Medium carousel, or there are some significant optimization to be had here?
Not looking to optimize white (or even Pink) sparks, I just want to have access to reliable fat stam/pow boost instead of crossing my fingers whenever the 3 goddesses event happen.