Not an expert as I'm new as well but I encountered the same issue with CB progression.  

My CB team through Easy and into Normal was Kael, FB, Urogrim, Apothecary, & UDK.  

As my gear improved and I got more speed I hit a damage wall. A Clan Mate pointed out that I was relying on poisons for all my damage.
Since the Boss can only have 10 debuffs at a time after I took 2 turns my team wasn't able to stack any more poison and my Damage was basically maxed.  

I dropped Kael (as you said, the low-level poisons will take up valuable deduff slots) and replaced him with Fayne (like Dracomorph she also brings poison, Dec DEF & Weaken). I replaced UDK w/ Godseaker Aniri and I'm 2 / 3 keying Nightmare CB.  

I wouldn't drop Frozen Banshee. She is amazing for me on CB. She hits like a truck and is consistently my top damage dealer (even though Fayne has more Attack). 

You may end up not being overrun with poison stacks. I'm pretty sure when Orn explodes a stack it falls off. You'll have to run the comp and keep an eye on it. 

If you have Apothecary, he helps a lot. Speed is king as a newer account and he has a good heal (it can crit).  

It also helps to have a Def champion to tank the stuns. If your stun tank can heal as well, it is a big bonus.  

Some other skills that I found help a lot are Leach & Ally Protection to extend the number of rounds you can survive. 

Just my experience. Hope it helps.