Tank swapping is not needed in hardmode if you have an assassin dps that can taunt during incinerate. I know this because that is how we kill them in hardmode.
To OP-If you have the bosses down to 15%, I wouldn't change too much in your strat. Get a bit more BH gear and download MOX Parser. Take your DPS to the bridge deck and dps the PVE dummy for exactly two minutes to see where everyone is with their dps. Once you know who needs to do more damage, work with them on their spec and rotations.
15% means you are doing a lot of things right. You shouldn't change your entire strat to try and get the last 15%. Change little things. You guys will definitely get them down soon. Just keep working on it and buying that BH gear.
Black Hole gear is not needed to complete story mode. If they are hitting the enrage, it is a DPS issue - simple.
I agree that you need to have your DPS on the Operations Dummy on the Ziost Shadow and see where your weak link(s) is / are and address the enrage timer issue where you should - with the DPS.