First off, I would like to say "good going" - you have done a lot and found answers that work for you for IBS. I am going to get back into herbs now - I remember Slippery Elm was helping me before... thanks for the inspiration!!
As for FIBRE, I have a theory that a lot of the IBS pains are from a buildup of GAS, it might only take a little bit in the wrong places - "bloating". With that in mind, I think that fibre causes more bloating, and gas, than is worth it for what fibre does. i think fibre is over-rated. I have good, large, easy bowel movements without eating much fibre, and I gave up on fibre supplements a few years ago.
As for dietary aids for IBS, I have found a few good ones... and maybe it is the OILS and not the fibre that is helping me - so I vote for FLAXSEED,Pumpkin seeds, Sunflower seeds, Olive Oil, and here it comes: HEMPSEED HEARTS [hempseed with the shell taken off = "hearts"].
AND - I suppose you have tried to eliminate CARBS from your diet? I find they are really bad for my IBS, and my Fibro/Fatigue. Just a little bit of regular flour, even whole grain flour, or sugars or other carbs... even veggies - I eat just a little bit of veggies because they are good for nutrients but are a source of carbs.
My special PANCAKE: no flour, maybe a tablespoon [really!], but grind up the above mentioned oilseeds, and use olive oil, and a milk substitute [Almond Mild, Hemp Milk] [if cow milk bothers you]... and I sprinkle hemp seeds on the "2nd side" before it is flipped.
This is an example of how I have elminated carb foods and still manage to eat something!! [These are OUT: Potatoes, Rice, Corn, Flour, Pasta, Bread, sugars].
Good luck! [sorry I am so wordy]
Karlin