Or maybe just simply a slight HP restoration on parry?

However lore-wise it wouldn't be able to be explained, that would simply do the trick.
Easy to implement, easy to understand and clarify. Tick per parry wouldn't be much, and both at bosses and trash pulls it would be simply usefull. Becuase there is never too much health.

You may say, that it's pointless, becuase tank is mostly on full HP during boss fights, but that's not something that we should be concerned about, right?