When my guy got it i ended up just doing a complete reduction back to just hay.
Then i added pieces in one at a time. For my rabbit it was pellets that did the damage, but I know some people who have rabbits that struggle with fresh veg of any kind.
Akin to ibs on humans, its not always obvious what the trigger is because it seems it can be different for everybun.

I will just echo Dface. ISC is often caused by too little fiber or too rich food, or a combo of both. It can be the pellets, sometimes a switch of pellet type can help but more often it’s more about reducing the amount.

Then there are buns that cant have but very little vegs or fruit or react to a certain veg or fruit. I had a bun that couldn’t have carrot or he’d have soft cecotropes. Some buns react to banana etc.

If your bun will eat more hay if you reduce pellets, that would be great, but never do any rapid radical changes to a bun’s diet.

There are prebiotics that can help get the tummy microbiota in better shape. Benebac is much used by bun owners. ISC is believed to be about a (minor) disturbance in the gut biota.

How much does he weigh? The HRS recommends 1 tablespoon per kilo bun for healthy adult buns. Cutting back further wont harm his health provided he eats a good amount of hay. To keep him from starving himself, you could perhaps divide the daily ration up into two servings, unless you’re doing that already, of course.

Giz is a big rabbit, which of course makes it difficult to weigh him at home. If he’s a good weight, you’ll need to keep an eye so he doesn’t lose too much weight since pellets are very calorie dense and cutting back reduces the daily caloric intake significantly.

Some buns are prone to sticky poop. It doesn’t seem to affect their quality of life very much, but it’s of course inconvenient for the human.

Sometimes sticky poop is a matter of the bun not being able to reach his cecotropes or clean himself, it can also be about not being able to notice when there are cecotropes due to impaired nerve signalling.

Bringing pellets down could help if he makes up for it by eating more hay. The importance of hay for rabbits can’t be over-estimated.

To avoid choking, you could perhaps switch him to a brand of pellets that are bigger in size. Some buns have problems with small pellets. The bigger varieties are less likely to cause choking. Some people with very choking-prone rabbits soaks their buns’ pellets before serving them, but not all rabbits are willing to eat pellet mash.