Day 10 doesn't seem to be the one with the best payout (it's an avarage x50), but I actually don't think it'll be necessary to cut off any race day anyway, since it's a 10-day event, while events on Nutaku run for 14 days (or in this case: 13 days).
It's also possible for Nutaku to handle the event slightly different in the first place: put the racedays on different "pages", kinda like we have for the gacha-events, and let the players move to the "next" day whenever they want. Release day 1-6 for Phase 1, and day 7-10 for Phase 2, and it shouldn't be a problem for people to clear it.
Main question is: will we be able to use the cheat-sheet from DMM, or will they randomize the winners?