I had FMT at a clinic in Stevenage, Hertfordshire around 2015/16. It's quite costly and sadly did nothing for me. I would have thought if it was a "game changer" it would be above the radar by now.
I have tried a lot of experimentation around the biota from various angles as well without any success or even a change (worsening or improvement either way).
Before FMT was a real thing, but it was noted the bacteria in the intestines could have impact, I experimented by attempting to entirely wipe out (sterilize) the bacteria in my digestive system.
Following other protocols that were used to sterilize mice for biota studies, I took a cocktail of multiple different antibiotics (I remember there were 4 types, I recall ciprofloxacin, metronidazole at least), and eating only very small amounts of controlled 'safe' foods for 2 weeks, with fasting entirely for the final 4 days while still taking the antibiotics.
As I exited my protocol I introduced a steady diet of probiotics that were considered to be 'good' eg kefir, yogurt, pickles, etc. along with only turkey meat for a week after that.
I felt no improvement or even a change.
I did have extreme exhaustion for the duration of taking one of the antibiotics. I removed these slowly, one at a time, one every couple of days. I noted that the metronidazole exhausted me in a different way than the CFS does, and that quickly diminished a day after stopping that particular antibiotic.
The results? I lost over 35 pounds, but the CFS fatigue never got worse, never got better, and stayed on unchanged.
Bottom line - I would expect some level of improvement or worsening, even if slight, after or during this experiment, if the bacteria itself were a cause. Instead the only change was a different fatigue that seems to occur every time I take metronidazole.
After that, my personal determination is that the gut biome is a red herring in our disease. Studies have noted that there are many wide ranging bacterial changes during any sickness, but in all the testing done so far, nothing has been shown consistently to introduce improvement due to changing the biome back toward recognized 'safe' biome standards (those that well people have).
I think the bacteria thing is just a downstream effect. I would love to be proven otherwise though!