Sure thing, glad it helped! As for refreshers, generally all of them are worth something because refreshing is fantastic. Different dancers will have different roles, but generally speaking it’s finding their unique support.
Sylvia would want +HP boon and as much HP stacking as you can to run Infantry Pulse in C. That speeds up the specials of all your infantry units with less HP than her, and is a really common and effective AR defense team strategy with AoE nukes like Lilina, or just outright nukes like Sonya.
Tethys has really good Res, and can run Temari+, Sabotage skills, or Ploys to debuff enemies with less Res than her. Temari+ and Sabotage can be good for AR offense, since enemy teams tend to bunch up.
Reyson has passive healing in his weapon, and 3 movement when transformed, allowing him the best movement of any dancer, tied with Leanne. He’s a very strong combo with Tibarn, and if you decided to grail Naesala and some day pull a Leanne, that’s Bird Spam, and it’s useful for…really everything.
Again, it all depends on the team they’re supporting, but every dancer is good if you’re keeping their strengths in mind. The only downside is, no dancer scores particularly well in Arena without the corresponding Duel skill. Which…are really rare. The previous post gave some good pointers on Arena scoring, so I’ll just echo their thoughts. Pick three favorites with a BST of 170 or more and work on merging them up first and foremost. The merges count for more than anything else you can do with them, and your three Arena core units should be your first merge priorities.