Another vote for Kiril. You’ll use his attack and defense buff everywhere, and you definitely need a healer in that lineup. After him, I’d do Hansel for the fast mana control. Then, if you pull Wu, I’d do him as your yellow- or else Chao works too for the fast mana cut.
Then you’ll have your first full rainbow team of maxed 4*, and can start on maxing Grimm for the best pulverizer, Sabina for a deep immediate heal and dispel, Triton for a fast sniper that’ll increase your healing received, and Kashhrek if you find yourself needing a solid defense and war tank.
If you finish all of them before you pull any new 4*, you could finish those 3* for events and filling out your roster for war and color-stacking. Brienne and Bane are both very good 3*. I’d also keep Valen, Ulmer, Gato, Gunnar, Nashgar, Hawkmoon, Azar, Rudolph, Berden, Belith, Mnesseus, Namahage, Gan Ju, Kailani, Melia, Balthazar, Tyrum, Gill-Ra and Chochin too- if you happen to pull them.