You almost certainly have SIBO (the root cause of most or all IBS). The fact that you have alternating diarrhea and constipation means that you will likely test positive for both hydrogen and methane with a lactulose breath test, meaning overgrowth of both bacteria and archaea in the small intestine. All of the things you mention are related to that. Vagus nerve issues can contribute to the cleansing of the small intestine, as would Insufficient bile or stomach acid (I got it 10 years ago after a doctor put me on excessive PPIs which greatly reduced my stomach acid for a prolonged period).

It is possible that your dietary changes put you on a low FODMAP diet which is part of dealing with SIBO, though it's not a cure by itself.

What is super interesting to me, as someone with SIBO and also histamine issues, is that going on a low histamine diet reduced dysbiosis. This suggests to me that histamine itself might worsen SIBO, unless they were coincidentally going on a lower FODMAP diet. However, many of the foods restricted on a low histamine diet are not restricted on a low FODMAP diet. This study may be useful for me.

Incidentally, you can also reduce the issues with antibiotics, herbal antibiotics, or an elemental diet, though the improvement often isn't permanent if gut motility is Insufficient.