It’s the boring answer, but I would suggest picking a few of 3 and 4*s and get them leveled as up before sinking a bunch more resources into your 5s.
Maybe merlin, Peter’s, Grimm, li, and gormek. In 3s I really like kvasir and Rudolph
That being said, it’s not a bad plan to have 6 healers for wars.
Kiril’s costume then Grimm, Ametrine then Sabina/Merlin, Mist then Li Xiu, Melendor and costume then Peters, and Boldtusk then Shadereave/Sir Lancelot, to name a few. Try leveling them using same colored feeders. If strapped with rare ascension mats, work on your 3* heroes like Bauchan/Rudolph, Kvasir/Kailani, Frosty, etc.