Also if I remember the fight correctly the tank swap at 9 stacks of Gastric Bloat will come in the middle of the period after the third inhale. This would mean that the first tank can use his strongest defensive cd for the first half of that period (when festergut does double the damage) and the second tank tank use his strongest cd when he takes over. Only ever tanked as a DK so I would use Icebound Fortitude which depending on your defense would decrease the damage by closer to 50%. I use use my other cds between inhale 1 and inhale 2.
I takes a bit of practise to get it right and except for all the other things like spores, tank using their cds at the right moment and the total raid dps actually beeing high enough I think this fight is most challangeing for the healers since they have to shift their focus from healing the raid to healing the tank more and more, and then back again on raid when Festergut exhales.