I'm not sure if this will turn out to be quackery or snake oil, but the Super Gut book by William Davis makes some believable claims as to common causes of digest issues (along with solutions).
The key I found was at about the 2 month mark things seemed to be getting worse, but after that a dramatic improvement. I was able to stop the supplements after the 3 month pack, though 6 months on there was some regression so I'm taking another round (which seems to have improved things).
It might not be possible for whatever reason to sustain the culture in my intestine, but it's been the first actual improvement I've had in decades.
Available as BioGaia, I recommend trying it if you have IBS symptoms.
The first stage of resetting, as described in the book, was to rebuild the lining of the intestine. Forgive my imprecision in describing it, as I have forgotten the details and don't feel like digging. The issue is that many of our foods are made with emulsifiers (baked goods, ice creams, etc.), and emulsifiers cause thinning of the intestine walls. That in turn creates a less than optimal environment, causing losses of good bacteria as well as increases/movements of bad bacteria.
Fortunately that first stage is pretty easy - making clove tea with a couple other ingredients daily. I actually found it pleasant after the first few days. And then I read about clove tea and discovered that it's already a big thing, but where I grew up we've never heard of it!