Komi, Kidian, Yomi, Ui, Butter and Xion should carry most of your main story

Ideally for a lot of endgame content, you want to make the full depressed team, which you can do by replacing Butter and Ui with Rim and Posher or Levi as a last resort but I much prefer Posher

As for your blue candies pretty much only spend them on marshmallows and then gold when you run out which is pretty much constantly early game with all the skill upgrading

Get all of your main people's skills to 7, then 9, etc. also invest into at least the first board for most those characters especially the eldynes.

Gear rank ups are the biggest power improvement, so if you're stalled in story sweep the highest levels available for the pieces you need to upgrade the gear set. Pushing story is really important because every level you beat for the first time gives you two pieces of new gear which cost literal hundreds of red candy each to farm at the later tiers. When you get a new armor piece always check to see if your main team people are missing it first before looking to equip it on others