This biofilm you are talking about, is it visible on stool?
I recently got some KAL brand Stevia and had lots of it with my homemade lemonade. My stool was looking terrible. It was covered in some film (not mucus). There was even white slime coming out independent of the stool (after the bowel movement) so I assume it's from further in the small intestine. I suspect that KAL brand Stevia is cut with something real bad since I don't see anyone else complaining about using Stevia.
When I accidentally ingest less Sugar with some meal I sometimes see not film on stool but smaller white threads from stool in the water. That makes me think it's not mucus but a fungus. When that happens I usually get needle-like pains in my intestine (I assume fungus growing rhizoids) and that gives leaky gut.
I am currently avoiding sugars (including lactose) and grains. I was wondering if this is the sort of biofilm you are talking about, maybe I should try your technique?