Off the top of my head, I think a way to allow DRK to spend its own HP on attacks would be that any attack that costs HP also grants barrier HP equal to the amount of normal HP spent with there being hard caps to the amount of HP you can spend, and barrier HP you can gain. And perhaps additional self-sustain tools can be accessed if the barrier HP gets broken. However it does not solve the problem of a healer pumping a DRK with heals, and letting them go ham, but is a direction that can be taken.