One thing we may add to the microbe stage along these lines is a “straw pilus” which can be used to suck out nutrients from other cells, that would allow for a parasitic lifestyle, if you can get close enough
In terms of later in the game I really like the flood however I agree it’s a bit hard to image. I’m not really sure it’s possible to imagine building a society without collaborating individuals. Though I guess it might be possible to have a caste system, for example, where one caste is a humanoid and the other caste is a fungus type thing which grows into trees the humanoids can get food from and live in or something. I think that sounds kind of crazy and also I don’t particularly see a reason it’s not possible.
I was listening to an In Our Time about parasites and they said that apparently 50% of species on Earth are parasitic which is much more than I thought.