Make the Yor require Nanites... or something that is not also a strategic resource that is already crazy hard to get cause everyone needs it for production already.
The Synthetics used to use a different resource but people complained. I am thinking maybe we should have them use Promethion instead.
Whatever it is - and nanites are a reasonable choice, but they have potential elsewhere for much more - grant it on or near the synthetics' home world. Might also up the chance of the same nearby. You can't fall back on the "random is fair" idiom. A dependent race that arose did so because the resource was available. Chance is defeated by fact.
My optimum solution is a resource (let's call it Fibrillium) that is useful in high capacity computing - and in the brains of synthetics - but nothing else: a new resource. Randomly distributed through the galaxy (except for the location of existing synthetics). If no one else can use it, they can at least mine it - probably accidentally - and have it make a dandy trade good for synthetics.
There have been several suggestions made that the AIs (in this case synthetics) not target global conquest, but target something like resource access. Synthetics want Fibrillium, not death to all meatbags.
