Throwing suggestions here.
1st.
Every rank 1-4 adds something like 2 or 2,5% more Innate Deathsave to DB and Group DB spells making DB have 38 or 40% save with rank 4 tome.
2nd
Dunno if this is possible but make Murk Beads heal more or make it have more charges.
Atm according to wiki it heals...