So maybe they're "zombie hording" as is another suggestion.... why?
As a direct combat strategy, agreed, this doesn't make sense - they have plenty of surface and ship combat forces already.
Sabotage, infilitration, maybe even just encouragement of self-destructive paranoia, there's perhaps more options there where a controlled human could get into places a Thargoid could also get into but the Thargoid would raise a lot more suspicion.
But - in terms of the timeline - it's all a bit weird.
- the Thargoids have been capturing people for decades and there's a hint in one of the HIP 22460 wreckage logs too
- there's fairly strong signs that capturing people may have been, if not the only purpose of the Titans, one of their major ones
- so whatever plan they had can't have relied on anything they found out from Seo [1]
- they clearly had a decent idea of the size and shape of the bubble just from where they aimed the Titans
- they clearly had an idea of which systems were considered important from Raijin using Shinrarta as a distraction
[1] I do wonder if this is again a way to avoid people wondering
how they know stuff - in the same way that Wycherley had people follow the Hesperus' trail to make it slightly less obvious that he already knew about the Proteus site and the potential of Guardian weapon technology - to therefore conceal that they've been perfectly aware of a lot of this for quite a while now.
"these were the beings that killed us, and this is how they reproduce, they don't have queens producing lots of simple drones like us, they aren't a hive mind"
Taking that approach further, the pyramid may not be a description of human reproduction but a rough sketch of human social hierarchy
- you have a small number of leaders
- who control a larger number of subordinates
- who each control their own subordinates
(and there's more than three levels, of course, but maybe that's implied - again, as distinct from the hypothesised hive mind which is either flat or only has two levels)