Let me first state that I'm against "rapid transit" (sounds uncomfortable). I also suspect that:

1) mTOR response (essentially insulin and sugar metabolizing signals that tend to make us fat or skinny) may play a big role in gut biome.

2) A large python has long transit time, doesn't care what it eats, we developed a brain to be smarter as we have shorter transit time.

3) Gut biomes are intertwined with immune systems, especially early in life?

4) Fiber = Good, except as you point out, what kind of fiber and how much? I always suspected that since most of us eat too much, fiber was our gut's braking mechanism.

5) Gut biomes are probably not "one size fits all"

As a society, we watch Oprah and dutifully gang up on probiotics and fiber when Dr. Oz etc telsl us so, yet many questions remain. I always wondered "does a mild antibiotic become a probiotic and vice versa?", a kind of "glass half full/empty" argument. My take:

https://scottvern.medium.com/an-imagined-interview-with-the-gut-brain-bb52d2af3d45