I would just drop dates altogether. Sounds like you react to them as badly as I do. They're sexy on paper. High in nutrients (magnesium etc), dry, high in sugar, portable, calorie dense..... Just perfect. But without doubt, the worst food out of everything for my body to tolerate. Along with bananas, give me the most profound dip in mood, and puts my gut into turmoil.
The only fibres I seem to tolerate well are carrot and bamboo shoot. Which I will now not be deviating from.
With another fibrous food, I am currently at the point of giving up mushroom now. It definitely gives me a bad stomach/gut ache (and I've tested all parameters I can possible think of.. Cooking times, trying every type of mushroom, eating organic, eaten with low/higher amounts of fat, trying different food combinations, etc). A real shame. Gonna miss mushrooms/cheese (that was becoming a real favourite, but the physical effects hours later aren't fun).
A simple diet of animal products + sugar may be your safest option.
Even after 2.5 years of PUFA restriction, I'm still in the same boat as you (my gut gives me hell with fruits, starches, etc). But like many here, on the rare occasions I eat a slice of simple white bread = effortless digestion. I would agree it's definitely the fibre. A happy gut, for me, does make a big difference to a number of things (mood, energy, libido, etc).
Do you do dairy everyday? I've often thought it might be the dairy that is making many here so intolerant to things. Pretty sure my gut was way stronger than this, pre-Dairy (which I have done everyday for the last 2.5 years). Maybe a sensitive gut is a sign of health. It's hard to go dairy free though, when you have such low faith in starches and fruits (I've completely lost confidence in them).
Can't eat mushrooms aither. They cause awful bloating. Can't imagine how they could be antiseptic or anti-endotoxin.