I absolutely hear you about the problem with fiber. I have the same issue. There are times when I can eat a little more, but definitely times when I can't. That includes both soluble and insoluble fiber.
But I can handle butter. In fact it is the one fat that definitely doesn't upset my gut. Butter contains good amounts of butyrate.
But I am not lactose intolerant, so if you are, that may not be helpful.
Apparently beans and legumes have good amounts of butyrate too. Now those foods can be real devils for my IBS-D, and I avoid them frequently. But I did find a little hack that works for me. I can eat chickpeas (garbanzo beans) from time to time; and the amounts can vary, from 2 dessertspoonfuls, to 1/3 of a can, drained and washed.
The hack I found is to peel every single one I'm going to eat. Then they don't cause me any problem, but those skins are total insoluble fiber and will cause me upset if I eat those.
There's some more info here about foods that contain butyrate (but many are high fiber foods) The only one that isn't , is butter. Fortunately I am safe with that. But I don't know if you can eat it?
https://nutritionyoucanuse.com/food-sources-of-butyrate
It's also possible to buy short chain fatty acid supplements (GABA supplements) I haven't tried those, so don't know if they can irritate IBS or not.