This is insanely anecdotal and doesn't hold any scientific integrity, but:

I ate a lot of raw, sometimes fermented, vegetables and fruit. Generally high in fibre. I avoided foods I'd need to wash much because I was trying to fill my system with a diversity of bacteria. A bit of dirt was part of the goal. I ate stuff out of my garden, from a little local organic farm, foraged foods like berries and mushrooms, etc.

No idea if that's what helped, but I'm totally fine now. Whatever I did before wasn't working, or wasn't working quickly. Once I went to my mostly plant-based "dirty" diet, so to speak, the improvement was rapid. But it didn't happen in isolation, either. I also reduced my intake of animal proteins mostly, which would probably be a significant factor in digestion. Eggs would reduce the presence of sulphuric gases especially.

I'm not a believer in pro-biotics (they don't seem to support diversity, which seems essential), but I believe people should eat a lot of diverse bacteria in their foods. Things like wild mushrooms, plants grown in your yard that aren't extensively washed and peeled. Don't cook everything to death. Let some little critters into your system and let them do their thing. Assuming you don't get the particularly bad ones (usually easy to avoid if you know how the food is grown), I can't think of easier ways to introduce the diversity required to properly populate your GI tract.

Again though, I have no idea what I'm talking about. I'm just some guy who eats dirty carrots in his yard.