Personally I think just removing a cd from abyssal drain and giving it a cast time again would be a good way to fix the drk sustain issue without it being op. Make it like a 3-4 cast so you have to factor in that you'll take ticks of damage while using it.

Honestly I'm fine with TBN it rewards you by refunding damage you could of spent that MP on. That's the reason it uses MP, if they got rid of the MP usage they'd have to remove the break reward then it just becomes a shitty Divine Benison.