Yeah, the fibre thing ... I have spent so much time eating vast amounts 50g/day or more. I think it was useful in the short term in that it produced butyrate that could fuel my body and brain, but of course it also produced propionate which seems to be a problem.
Thats why I suggested a keto approach, you would make your own butyrate and obviate much of the need/benefit for fermentable fibre.
I kind of stuffed up on this when I tried a GAPS diet to try to get rid of SIBO 4.5 years ago. When I stopped eating FODMAPS I stopped making SCFAs and my brain stopped working too.
But if you are getting by on a low fibre diet, this is perhaps not an issue for you.
Intellectually I can see that the flipside of all the stories we see and hear about how important your microbiome is in its interactions with your immune system and its production of neurotransmitters and so on is actually an argument for saying if your microbiome is treating you like crap you should ditch it. That is what a fecal transplant is all about.
Plant foods have been so much a part of my diet that I just find it hard to imagine taking that step.
Personally I have been undereating protein most of the time (0.5 -0.7g per kilogram bodyweight). I discovered A) that some combination of ubiquinol a ketogenic diet (or maybe just high dose MCT) and Mk4 relieved my 24/7 lymph system pain; B) that eating more protein brought the lymph system inflammation back; and C) that having more protein made the rest of my body more painful, but came with improvements in PoTS and a better sense of my body.
I do not know why protein leads to lymph system swelling.
I do know that over the years I have got used to not feeling much of my body, and seeing my legs for example moving by reflex as if they were in pain without feeling anything. I wonder if the low ceiling on protein consumption lead to a deficit in signalling molecules and if such a deficit could what is caused part (or all) of my PoTS and that lack of feeling in my body.
So I have been reading about this subject and can see the logic in it, and wish you luck.