Personally i think TBN is a tool to especially reduce big hits, so a 10% dm after it breaks seems unnecessary bc you can use oblation after it breaks there u have your 10%dm. I think a drk should be able to increase the effect of tbn like a warrior can increase his shake it off. Like using shadowbringer will increase the effect of tbn by 5% per use. This would also change make the drk (hopefully) some more complex because you have to decide whether you want to use your 2x shadowbringer in the 2minute raidbuff window or you want to use it to increase your mitgation. I also think that after the effect of tbn is over it should give dark arts regardless of breaking or not. When tbn breaks however it should let your next edge of darkness crit.

Oblation should be redesigned to worthy small defcd like every other tank has (like holy shilton and so on), somethink like 15%dm and an excog like gunbreaker ( would also massively help with living dead).

There SE fixed drk for you