I think you're looking at possible drug side effects from some of the meds your vet used, as the cause for the gut problems. Drotaverine and metamizole, as far as I can tell, aren't used in rabbits, so their safety for rabbits is unknown. If I had to guess, because this occurred when your vet first prescribed the drotaverine and ranitidine, that there's a good chance the drotaverine could be the cause of all this.
Originally your rabbit had something called GI stasis, where he stopped pooping. This was likely due to his molting and ingesting fur, which can slow down the digestive tract. And because he has a high carb diet and eats very little grass hay, then he wasn't getting sufficient fiber to help move the ingested fur out of his gut, so his gut motility slowed down and stopped.
The usual treatment and meds for a minor to mild case of GI stasis is, a gut motility med like metoclopramide or cisapride, ranitidine(though this may not be used any more because of recent problems discovered with the drug), sub q fluids if the rabbit is dehydrated, and the pain relief/anti inflammatory meloxicam. Then a few other meds might be added in, but those are the basics.
https://rabbit.org/gastrointestinal-stasis-the-silent-killer-2/
So if it was one or more of the meds that have caused the mushy poop problem, because it damaged the nerve function in the digestive tract(which is one of the main things for proper poop formation and movement), about all that could be done at this point that I can see(just my opinion), is to try and help correct the as much of the disfunction as possible, with a strictly regulated diet that is high in fiber and low in carbohydrates.
One of the best things to help correct digestive problems in rabbits, is indigestible fiber from a good quality grass hay. This helps promote good gut motility and helps to restore the imbalance of the microflora in the gut, which balance is absolutely essential for rabbits to maintain good health and proper gut function. This high fiber low carb diet would basically be following the recommended treatment for cecal dysbiosis in rabbits, that causes mushy cecotropes.
https://rabbit.org/intermittent-soft-cecotropes-in-rabbits/
So starting with free feeding a good quality grass hay. I can't tell from the photo how good the quality of that hay is, so you'll need to determine if you think the quality is fine. Iif it's not a good quality hay, I would suggest getting a different grass hay. It absolutely has to be free of mold and never been wet, as mold can make rabbit very sick, even cause death. Signs of mold are white and black spots, waxy appearance, white dust, dampness, sour or musty smell.
Good quality grass hay should be a mostly nice green color, and it's best to be at least half to three quarters leafy strands(contain the necessary protein and nutrients), and the rest the crunchy stems(high in fiber). Then this hay needs to be free fed, meaning your rabbit should never run out of it and it needs to be topped up fresh at least twice a day.
Picking good hay for rabbits
I would remove all of the other pellets and treats and only feed the Versa Laga Complete, though at a reduced amount than what you have been feeding so that your rabbit will start eating hay. About how much in cup measurements, do you think you currently feed of all of those foods/pellets combined? And about how much does your rabbit weigh?
