Don't waste your time on any of the rares - with the exception of leveling them as part of the Rhazin fusion, as you've clearly already begun doing.
With regards to your epic champs - you have a fair number of pretty good ones! Kinda surprised you haven't got any legendaries yet - bad luck I suppose. But you've very smartly avoided most of the common mistakes people make.
I would get Khatun maxed - she's amazing just about everywhere until you get to Arbiter.
Ultimate Galek is quite good for Spider as well - good candidate to level for that purpose.
Fahrkhan the Fat is a really good CB team attack champ. Highly recommended.
Other possibilities:
Vrask is a fairly good healer for most fights, I'd argue better than Apo - although since you've already got apo maxed, it may not be worth leveling another healer.
Fayne is quite good for CB, but without an unkillable team she dies fairly fast.
Toragi the Frog can reliably put poisons onto CB, and has some good healing as well.