I'm back, and I've listened to Spare Parts. While it did have some of what I was looking for, it still retains the old-Who fascination with Soviet allegory, rather than pursuing a more "pure" critique of transhumanism. This manifests as being at odds with the description Mondasian Cybermen gave of themselves in The Tenth Planet: they asserted that they had pursued cybernetics to eliminate their own "weakness," which implied a willing endeavor based on hubris and existential dread. In Spare Parts, the Cybermen are mere worker pawns who were inducted into a conversion program with no understanding of the extent of modifications they would undergo. Tragic, but uninteresting. I've seen this already in "Rise of the Cybermen" and "The Age of Steel" (which I am aware are based on Spare Parts and not vice versa, but first impressions are a bitch to shake).
Frustrated, I found one other piece of Cyberman media: ArcHive Tapes: The Cybermen. It's pretty much audiobook fan-fiction by David Banks, the actor who played the Cyber-Leader, and while it goes on for too long, it starts with the skeleton of my perfect Cyberman story: the Mondasians were advanced humans, they moved underground when their planet was flung from the solar system, and as they relied more and more on cybernetics to survive, their science-worship and apathy towards the aesthetic opened the door for them to slowly discard everything of real value in themselves.
While a story of social degeneration spanning centuries would be a bad fit for episodic TV, I'm left with a vague fantasy of a classic Who episode cliffhanger: the Doctor and his companions, trapped in an underground city filled with eerily stoic jumpsuit-clad citizens, have spent the first episode in the 4-parter learning about the alien society's obsession with cybernetics and apathy for beauty and art: the Doctor, eager to see what the surface of the planet is like, finds his way to an elevator, but must hide from a group of engineers also heading to the surface to perform maintenance on sensor arrays: the leader approaches the elevator, and from under his arm he takes a helmet. The episode ends on a close-up of the lead engineer's face as he lowers a rudimentary version of a Moonbase-style Cyberman mask onto it.