If this turns out like Draig's game, rolling at advantage is USEFUL but not necessary, and takes up a die you could alternatively save for a revision to fix the problem(s) given by a bad roll.
Anyway, doesn't matter since I will hardly ever recommend rolling at advantage.
So, you don't understand the actual mechanics behind the bug system:
They do NOT engage in battle. Instead, they would be taking out civilians and hostile cultists not actually armed. We then call back the massive resultant swarm (Once they'd consumed an entire person) and convert it into, say, a couple of Genestealers or something.
New plan, however, because the bug swarm as it exited the corpse would be slightly obvious:
Heretic Plague
True to our name, we do in fact have the Microbe Division, made of up a hundred trillion crack microbe spec-ops teams, ready and willing to force anyone who ingests them to serve the Hive-Plague. To start simply, we have designed a modification to a single Genestealer, which secretes a compound containing a virus based on the Genestealer's DNA-injection system (That's what viruses DO, after all), which is, at present, only waterborne. This allows us to infect various numbers of people and gain immense numbers of recruits, very quickly, should the need present itself.
Greater Plaguemind (Does this have to be a Grand Design? I hope not)
A gradual adaptation to our Patriarch, focusing our hive mind on him and his will, allowing our pure-form units greater adaptability as the Patriarch changes their (Still beast-like and simplistic) orders as necessary. Not only does it tighten the Patriarch's control, but it also makes our hivemind stronger, binding our units together more than before, allowing for great levels of coordination against the machine-armed foe.