Spread, do not concentrate. This is the opposite of most Death Guard advice, and it follows from the arithmetic. Two characters standing together Afflict roughly one bubble's worth of enemy units. Two characters twenty inches apart Afflict two. Since your damage is counted per Afflicted unit, board coverage beats local density.
Spend Signal Pox every Command phase you can. One command point for a whole objective's worth of Afflicted units, at up to 30 inches, is the best rate in either detachment. Pick the objective with the most enemy units near it, not the one you want to take.
Choose your Plague for durability, not damage. Your free damage does not care which Plague is active, so pick the one that keeps your slow blocks alive. Rattlejoint Ague against a shooting army, Skullsquirm Blight against melee. If you find yourself wanting to change it mid-game, that is an argument for a different core detachment, because this pairing has no way to.
Walk your Terminators behind the Helm. With Helm of the Fly King, your main block is untouchable by ranged attacks from beyond 18 inches, which is most of the first two turns. Use those turns to get where you want to be rather than to trade.
Use Blooming Pestilence on your opponent's turn where the timing allows. It works at the start of any phase, and what you want is for it to still be active when their Command phase resolves Deadly Vectors. Read the phase sequence carefully at the table, and if in doubt spend it in your own turn on the Terminator block closest to the most enemy units.
Command point planning is easy here because both suites are cheap. Every stratagem in both detachments costs 1CP. Budget roughly one for Signal Pox each round, one for Blooming Pestilence, and keep one in reserve for Undying Spite in the Fight phase.