If you want to keep the DG codex as your only source, then I think the plagueburst crawlers are your only real option.
Alternatively you could add one or two more psyker characters. I don't have the codex but I play against my brother's DG regularly and I believe that DG have one or two psychic powers that can buff units and make them even more survivable. I know the two characters you have are already psykers, but having more would allow you to buff and still be able to throw out a decent round of mortal wounds each turn.
Nonsense is our Salvation