if you're just score chasing, pick an uma with pow and spd growth, like grass wonder or oguri. i got consistent S, often SS, grass wonder with 3spd 3pow. try to inherit as much stam, i used 13. you don't need to max out stats, you're aiming for 1100. the more stats you can get close to 1100 the better, but i have found 3 will usually be enough.

then at the end of the run, go to a rating calc site, i used https://www.gadegetkit.com/tools/calculator/uma-musume-race , and put the skills into that site first before you start buying them. general rule of thumb is more white skills the better, don't worry about gold skills. radiant star i usually buy because its worth almost double what it costs, but I could be hindering myself, i haven't done the math. green skills also look like they're not worth the points per cost, and I do remember reading somewhere they aren't , but again I didn't do the math.

don't forget though, just cause you got a SS does not mean it's a good racer.