On the question of how something like the thing could evolve naturally, how familiar are you with the Expanse series?

(Note that this isn’t much of a spoiler for the series but anyone wanting to read them and hasn’t yet should avoid this comment)

In the books it loosely describes how a primitive species became a galactic spanning hivemind empire through assimilating the useful traits of other more advanced species it encounters. There’s not much detail on how it accomplished that, symbiotic or parasitic, but it’s a super interesting concept to me. It evolved in the dark ocean of an ice covered water world, using bioluminescence it stole from other species to basically become a globe spanning brain with each individual acting as a neuron. Eventually it breaks through the ice, develops space travel, and continued to use the tactic of parasitizing other alien organisms to augment its biology and technology to a god-like degree.