If you still want to enjoy fries, get an air fryer and just spray sliced potatoes with a very small amount of olive oil. Then cook only up to temperature of 250F/120C to prevent acrylamide from forming (it might take 30-60 minutes to be finished).

I used to be hit hard by regular fries (was feeling sick for a day, felt them staying in my stomach for a while etc.) until I switched to the aforementioned method. Taste is almost the same, but digestion is unproblematic now.

I don't think it's that, as I'll go ham on like 2000 calories of potatoes and beans after working out and feel great. My thoughts are that it must be a histamine response to some compound found in grains and some spices.

Processed carbohydrates will by definition hit you hard. Added oil is also hard. Try intact grains, e.g. rice, quinoa, millet, barley, etc. Even whole wheat bread or pasta will hit harder than boiled wheat grains.

Added oil is just fat, not bad for you. Specifically since op mentioned keto.

But they're not talking about keto if they're eating fries etc. Added oil is not good - in excess fat builds up in the cells and is responsible for insulin resistance. With ketosis this is temporarily staved off, but you will still likely have insulin problems when you go off keto, it doesn't actually fix anything, just offer temporary relief.