I considered Vanguard/Promethian and Syndicate/Heritor for thematic reasons as well, but that led to awkwardness for the Dvar and Shakarn, as none of the others seem particularly appropriate for them. If a hyperdrive breakdown had cost me literally centuries of my life, I'd want to make sure I understood everything about hyperdrive going forward as well. Given that the Vanguard's origin story is that they are a unit from the golden age of the Star Union that has finally arrived due to technological difficulties with space transport, it at least somewhat fits. Mostly by process of elimination, but it's based on old Star Union hyperdrive technology. The Shakarn are a sinister secret alien race that has hidden in the shadows, ready to strike now that the Star Union has fallen. The Heritor are a sinister secret cult that has hidden in the shadows, ready to strike now that the Star Union is fallen.

The Kir'Ko are a psychic race that wishes to restore their ancient civilization (whether that involves restoring the queens, taking revenge or just trying to forge a new life) and Celestian is essentially a psychic religion that focuses on mending (or, for the cynic 'mending') society. It seems a logical continuation of their path. The Xenoplague essentially genetically engineers whomever it infects to create a new creature in service to the Xenoplague user. Specifically, the Amazon's backstory is that they are genetic engineers/terraformers who were forced to use their terraforming/genetic engineering skills to engineer themselves and the wildlife in order to survive. Because evil psychic slavers deserve evil psychic powers. As such, a technology based on hacking and networking units makes the most thematic sense for them. More broadly, Assembly is the closest faction (aside from the NPC Autonoms) to an AI faction, with deliberate 'Borg' overtones.

Because evil cyborgs + hackers for the win. More broadly, the Dvar are thematically fairly straightforward 'fancy machinery for the win' and Promethian is the least exotic themed tech. Because the forge of the dwarves should burn as hot as possible, and environmental problems are someone else's concern.

Age of wonders: planetfall best faction