I don't I'm afraid have any ideas that you might find useful but thought I would lend my own experience of this dilemma to the discussion if that's alright? Don't want to take over this discussion.
I still have this IBS issue - if that's what it is. Over the early years when the nausea was so very prevalent - various quacks and non-quack nutritionists were consulted and intolerance's were advanced as possibilities. Even tests 'proved' I had them e.g. wheat.
Diets were tried. Restrictive. Very restrictive. Nothing seemed to help and to be honest I got totally depressed and demoralised by it all - and it was bloody expensive to boot!
After 5 years I thought 'no more' and packed it in. In might actually have been less than 5 years - felt like 5 years - but it was several years. Anyway...
The nausea was resolved with Betahistine Hydrochloride in my case. Great. More than great. Although that didn't occur immediately - it took another 5 years of suffering with this symptom before an experienced ME clinician put me on that particular drug.
No. What I did was to decide that I have this problem with loose bowels, the diahorrea, the need for a toilet, the spasms in my bowel, and that nothing was likely to help me with it. There was no 'thing' that I could take or cut out from my diet that would help.
So I returned to eating what I liked, when I fancied it. I didn't observe set meal-times. I ate when I felt the need. I still do. And all the quack-remedies went in the bin.There were a hell of a lot of them.
Appetite is still 'out there' somewhere and I have no regular meal-times. Eat when I'm hungry and eat what I like. I am still a relatively 'big chap' I suppose but that is as much due to a inability to exercise as much as I would like as anything to do with my diet.
Bowels are still messy. Sorry. My lifestyle now is far more settled. I don't have quite the same degree of stress that I did - but I still have the symptoms - the problems - just less acutely I would venture.
It's a personal thing I reckon. I love bread. So I eat bread. Especially fresh-bread. I buy it part-baked and pop it in the oven and.... well you get the point. Nothing like that smell to stimulate my appetite.
Same with bacon... Ha! You're probably thinking 'You unhealthy buggar!' But I'm not. Not really. Pasta is my constant companion - wheat pasta of course and eggs are good. Natural binder after-all
The only thing - the absolute only thing - I steer clear of is apples (though I love apple juice and that's fine) and orange juice (though I eat oranges). These really go through me for some reason.
But in the main, my lifestyle change, my approach to food ('Sod it. I will eat what I want and tolerate the consequences.'), and my attempts to reduce stress - have I think helped considerably.
I hope that you are able to find alternatives to what you cannot tolerate. It's so important to enjoy - to gain some pleasure - from what you eat. Good luck