It seems like the microbiome in your gut is really important and fiber seems to be key in maintaining it.
(This is not the advice I would give for a person who is already suffering from IBS, but as a regular human, I'd take care to not let my fiber lapse for too long.)
My current favorite healthy high-fiber food item is something we've been calling "Mashed root vegetables" with parsnips, and now also chia seed pudding (on the Congee (and related Chinese medicine/energetics) page).
Lots of other foods are good: blueberries, dried cranberries (TJs has organic ones without sugar), raspberries, broccoli, potatoes, peas, quinoa, etc. I know legumes are generally high-fiber, but are frowned upon by the Perfect Health Diet folks; I think red lentils might be ok if cooked properly, but their toxic skins have been removed along with a lot of the fiber :/ (speaking of which, I’ve been enjoying Amy’s quinoa, red lentil, and kale soup this winter—kinda in the direction of chili, but only 4 or 5g of protein per can. 🥣 )
The Best Gut Health Cookbook has a chart with foods and their fiber amounts. (That’s also where the Mashed root vegetables recipe comes from.)
You can also supplement fiber. Metamucil is maybe the most well known, but the main ingredient: psyllium husks can be sourced pure and added to food or drink. Start slow, accompany it with plenty of liquids, and don’t take it within a couple hours of any medications or supplements.
If they’re kind of roughly chopped, I like to put them in yogurt (which I usually have with grain free granola + fruits + milk) — it has a similar effect as adding ground flax seeds. If it’s powdered, you can mix it into liquids. It will gel more quickly, which can have a strange mouthfeel, but it won’t float or stick to the cup in the way the flakes would.
As with anything, check with your doctor before starting something new — remember: I’m not a professional and your health is your responsibility.