Importantly, as HN folks seem to like supplements, you should be aware that most fiber supplements and additives are soluble [edited -- originally said "insoluble"] fiber, while all of these sources give you a mixture of soluble and insoluble fiber. This mixture appears to be necessary to get the full benefit from fiber—Dr. Lustig describes it very memorably as saying that the insoluble fiber creates a sort of “mesh net” and the soluble fiber “plugs the holes” in that net, which combine together to coat the inside of the upper intestine (the duodenum) and reduce early absorption of calories, so that they ferment more and feed your gut microbiome.
So, to supplement, supplement with snack-sized bell peppers or mixed nuts or fresh green beans or so, rather than a pill or a fiber shake.
This also means that it is very hard to actually control for and isolate that this was due to the fiber rather than the lifestyle. So maybe just find produce that you find delicious and eat it for deliciousness’ sake, trusting the fiber thing to work itself out?