It sounds like your pursuit to maximize your tank dps is actually costing you dps and potentially slowing your run down. Being in Sword Oath isn't a huge dps increase... especially in a dungeon where a majority of the damage dealt to mobs is via AoEs. By being in Shield Oath, you allow the dps to worry less about their threat and more about nuking. Furthermore, you take less damage, allowing your healer to deal damage as well. Their damage contribution is going to be greater than that of Sword Oath's contribution. With regards to Stormblood-specific dungeons, beyond Shisui, the tank begins to get absolutely slaughtered when pulling more than one group -- Doma and Castrum are strong examples of this. Your healer will spend more time spam healing you than necessary, sacrificing any potential damage they could be doing.

TL;DR: You're not doing your group any favors by staying in Sword Oath. It's a dungeon, not Omegascape Savage.