My usual priority goes as follows for single target: I open with diseases, following up with SS while building up for DT. The problem is that whenever I try to store DC charges to maintain DT's uptime I find myself without a filler and do only auto hits. I have been using PS and IT, but that doesn't help much either.
I think you might be missing Festering Strike; if I understand it correctly you spend Blood/Frost on FS, Unholy,Death on SS though Noxxic can give you a better priority breakdown.