Yeah, not turning to fire at adds straight away is something to learn as ranged DPS.
Is also something I noticed in phase 2. You can hold off shooting the adds that come after the 3rd drake until they've been funelled in past the rubble, so that Shellshocker with a rampant augment, or Dragon's Breath with augur hits them all. No need to waste 10-target AoE on 3 adds.
Cannibalise should be better in phase 3, yes. You'll have the extra Guts at a +30% buff, due to cannibalise and blood offering, so it will outdamage Frag Grenade, now that I think about it. Especially since the raid circles do not buff procs or afflictions of any kind.
(So if any DPS do still have procs or afflictions in their builds, get rid of them too. )