Have you ever tried adding in plain grass hay pellets to his diet? You could soften them like you do his normal pellets so they are easy for him to eat, and they would add extra fiber in his diet, which it sounds like he needs. He has soft poop because he needs more fiber in his diet. Hay pellets would help do this and hopefully clear up the soft poop issue completely. Plus it would help in preventing stasis episodes.
You could try a local farm store, an online one, or there is an online shop called galens garden that sells different types of hay pellets for small herbivores.
https://www.simplesystemhorsefeeds.co.uk/products/feeds.asp?id=1&name=Pelleted Forages
https://forageplus.co.uk/product/timothy-hay-pellets-horse-feed/
https://galensgarden.co.uk/shop/forage-pellets-cobs/
Or maybe try a different rabbit food pellet. Supreme has a grain free/no sugar added pellet, that may not cause the mushy poop issues that you get with the current pellet you use. Usually it is the carbs in pellets that lead to mushy poop developing.
https://supremepetfoods.com/us/products/selective-naturals-grain-free-rabbit-food/
Those would be the things I would try for a bun with a sensitive digestive system, that was unable to eat loose hay.