Saying “There’s no motivation for Genestealer Cult beyond the Hive Fleet” is like saying “There’s no motivation for Chaos Cultists except the Chaos Gods”. It’s thinking about it backwards, that’s their end goal, the thing they’re ultimately moving towards and doing things in service of - but there are all sorts of different things they can do to achieve it. They’re certainly not limited to one particular style.
In the same way you have various factions dedicated to the chaos Gods doing all manner of things, there’s also that with Genestealer Cult forces. There’s an Adeptus Mechanicus “Cult of the Bladed Cog” whose actions included assimilating Mechanicus technology, creating Genestealer hybrid servitors and Skiitari. The “Cult of the Rusted Claw” has a part of their religion that metal rusting is a symbol of liberation, and thus some have an adapation that lets them rust metal through touch. There’s the “Cult of the Twisted Helix” that spread through convincing the upper hive members that a serum they developed would grant them immortality, basically turning the whole thing into a medical science genestealer farm. There’s others that have infiltrated the Imperial Guard and are highly militarized. Others focus on the underhive members. There’s even one recently that got into a 3-way ideology stalemate with the T’au and Imperium fighting for control.
There’s just as much political machinations and similar variety as Chaos has, and considering there’s a Genestealer Cult that decided to swap from worshiping the Hive Fleet to instead Nurgle, they aren’t just locked into one thing.
Humans serving chaos tend to be indoctrinated in some way too, anyway, by Daemons, the Chaos Gods, propaganda etc.