We are running out of Helium.

>extremely narrow range of final goals for which flesh-and-blood humans are physically optimal

Not so quick there.  Currently Al  can't do anything without depending on humans.  I have yet to hear an explanation of how the AI rids itself of this dependence.

We do trade with our microbiome.  We feed it.  It helps us digest.

Our microbiome communicates with us.  It creates chemical signals that affect digestion and possibly your feeling of hunger.  The extent of the influence of your microbiome on your brain is not well known, but the pathways for that influence are. The gut microbiome has been shown to produce various chemicals and signaling molecules that can influence the function of the digestive system and the immune system.   It produces short-chain fatty acids that may cross the blood-brain barrier.  

Perhaps the analogy here is better than the one with ants.