Replacing plague dr with any decent stam set would be a DPS increase. Shoot, if you craft heavy hundings paired with your hulking, you could still tank all the 4-man dungeons and not notice the missing health.
I would think you could pull 10-15K DPS and still hold aggro. Your front bar could be your tank bar, and have a back bar for DPS. If the fight was tough, you could switch to full tank mode (which is usually only needed for vet trials).
Specifically for the portals, they are around 5K health. So, the question is: what is the quickest way to do 5K DPS? My guess is that most of your skills (shielded assault, poison injection, pierce armor...) hit for 2.5K. So you could: shield assault->LA->pierce. Or LA->poison injection->LA (then trust the poison dot will finish it off). Alternately, you could slot a lightning staff and: destructive touch->HA; this is actually easier, because you can spin/walk around searching for new portals while channeling your heavy attack.
The planar fight is a little different, because you need either ranged DPS or a gap closer . As a "damage tank" (holding aggro and doing a little damage) you are always going to be melee, so whatever you do for this fight will be different than other encounters.
However, for certain fights like this one (Planar Inhibitor on Vet WGT): https://forums.elderscrollsonline.com/en/discussion/comment/4407937 , people tell me a tank is useless. So I need to put out some of my own DPS.