Well, personally I'd suggest trying to CR Spica, and if you want to take advantage of the 3x XP, awaken Bashira and CR her as well with your mentioned dupes. Yuyu is pretty good as a replacement to Calliope, but isn't super needed if you aren't using her much already, and can likely wait (also needs her cost reduced some to make her viable as a low cost witch). Additionally, while gold units do still get the triple XP, it's not nearly as big a boost as it is for the platinum units, so choosing between Mehlis or Bashira I'd go with Bashira, and I'd pick CRing Spica over prioritizing Mehlis and Yuyu. That said, if you can 3* the G map reliably, today it gives 21 DC per run with its 100% drop rate, so if you aren't adverse to spending a few SC, you can likely get another 100 before the day is done (assuming at least 16 max stamina, you can get 42 DC per SC, not counting potential need to refill charisma, but you get more than twice the needed charisma in the time it takes to regen the stamina naturally).