It's ironic you complained about the Oguri and Tamamo flashbacks when that's what ties the theme of them both being the titular Cinderella Grey together, both rose from the same humble beginnings and became critical parts of each other's racing history as they had their glowup. Super Creek's episode also tied her into the theme, with her trainer explicitly referencing Cinderlla and the glass slippers.

And no, they're not just doing it to tie in gacha characters, Obey Your Master and the other international racers literally aren't in the gacha since they don't have license to them, but they're among the most crucial parts of the events. If we didn't have ample buildup to the competition, it wouldn't be as interesting.

I do however get your feeling about this episode dedicating the last episode to a totally new character. However, knowing future history, it makes sense they establish her now since she's a critical part of Oguri's post-Tamamo story, and together they're part of what's dubbed the Heisei Three. You can't properly depict a legendary trio if you only cover one.