I'll just make it quick for your short/mid-term goals:
- level Bashira and Iris some more (to around CC40-50ish)
- raise another healer, Bernice, Yuyu, Marius, Khuri
- buy Spica ASAP and raise her
- raise some duelists: Lyla is the obvious choice, I suggest Claudia as well since she's a princess
- raise a third healer at some point
When you did all of this you will have a solid core team that will allow you to do well in most events. Then you can start refining your team by leveling more niche/situational units and AW some units.
Most units were already covered by lolix, but just a quick note about Khuri: She's one of the most useful silvers in the game. It should be noted though that Nutaku is/was supposed to get a vampire hunter event, so you might want to hold off leveling her for now.
Sophie is amazing, but as an angel she is quite reliant on her skill. She has the same skill as the silver angel. You can trade a valkyrie and 100k gold for a silver angel in the trading post, so you could do that later on when you saved up some gold to save some rainbow fairies.
This is a common misconception among new players. Fact of the matter is you will have way more than 15 usable units in no time and you will have to adapt your team depending on the missions, so you shouldn't worry too much about figuring out your "main team" or whatever.
Grats on mincosting her, but unfortunately Monica's skill changes when you CC her. Skills that change upon CC reset to 1/5. For units with changing skills it's advisable to CC them at level 30 before feeding them to a CC'd main unit. Not the biggest deal, just something to keep in mind for the future.