I'v been on this train for a long time - diagnosed with IBS at 18, cleaned up my diet and saw some improvement but it's always been hit or miss especialy if I'm under stress - eating a lot - not sleeping enough etc. Problems can range the gamut from bloating, to constipation to clearly underdigested food etc.
I've been on a powdered, high strength probiotic for about 8 weeks now along with a liquid calcium/magnesium(1:1 ratio) supplement. Both of these things seem to help a lot with general regulation.
Other things that I've found have helped:
COOKED green veggies - raw veggies tear me up, lightly cooked (steamed, stir fried etc.) greens, broccoli, cabbage seem to help keep me regular and keep bloating down. The only green stuff I eat raw is romaine lettuce and avocado which seem to help as well.
Limiting eggs to one serving a day - it doesn't seem to matter if it's one egg or a half dozen, I can handle one serving a day with no problem - if I go for a second egg based meal I've got bloating and heart burn.
Limiting sugar and alcohol
Backing off on nuts/nut milks if I'm experiencing problems - I typically consume these every day, but if I start to feel "off" I'll lay off for a few days, otherwise they seem to make things worse
No casein - casein does NOT work for me, including casein/whey blends. Cheese and yogurt are usually fine, but casein based protein powders are absolutely no go. That said, I tend to limit dairy all together, it's fine in small doses infrequently, but when I rely on it I can expect bloating or worse.
I've never had good experiences with fiber supplements, but I definitely feel better with more fiber from natural sources, especially from a lot of different sources - flax meal, apples, oats, brown or black rice, cooked broccoli/spinach/kale etc.
Hope some of that helps, good luck!