Yeah, I would definitely suggest starting with completely removing the carrot and pear (and any other high carb food) as Diane R mentioned, as those high starch/sugary foods are the most common cause of mushy cecotropes. Or if immediately removing them from the diet is too distressing for your bun, start to gradually reduce them until you no longer feed it or you're just feeding a very small bit of it.
Then I would see how that goes and if there are any improvements in the cecotropes. If you don't see any improvement after a couple of weeks of the amount being significantly reduced to just a small piece, I would suggest removing both from the diet completely. Or if you already eliminated both completely and there's no improvement after a couple of weeks, then you need to start looking at the other food. Possible other culprits could be any of the other veg, the fresh grass or other plants she eats while in the garden, possibly the pellets, or very rarely the hay.
If their pellets are a cheaper quality pellet or a muesli mix, those are more likely to result in digestive issues than a better pet quality plain pellet, because of the higher carb content and lower fiber that is usually in the lower quality pellets. If you are feeding a good plain pellet, even though it's possible they could contribute to mushy cecals because of the added sugars and grains (though they contain a lesser amount), because you're only feeding 1/4 cup once a day for both rabbits, and big rabbits at that, I think that would be less likely to be causing the issue. Though still can't be ruled out completely.
Their hay is the least likely of all their food to be a cause, provided it's a good quality grass hay that hasn't gone bad, isn't contaminated with anything, and doesn't contain weeds that might be the cause. But if it's alfalfa hay or is a very rich and soft early growth cut of grass hay, then it's possible it's the cause, as those hays can cause digestive problems for some rabbits, or can cause an over production of cecotropes due to the excess nutrients and protein, particularly in addition to getting fresh grass each day, which tends to be more rich in nutrients unless it's a longer more mature growth. If you are feeding alfalfa or a rich early cut grass hay, I would suggest gradually switching to a medium coarse good quality grass hay like second cut timothy, orchard, meadow, etc.
So if it's not the hay, and removing the carrot and pear doesn't cause an improvement, then I would try removing the cucumber and celery next. If there's no improvement from removing those after a couple of weeks, I'd try reducing pellet amounts last. Unless you're feeding a poor quality pellet or muesli, then I'd suggest gradually switching them over to a better quality pellet like science select, etc, right away. That alone may resolve the issue. I've had poor quality pellets cause immediate digestive issues with only the smallest amount being given, particularly with my most sensitive buns.
Rabbit diet and feeding
If you've tried all of these suggestions and there's still no improvement, lastly would be looking at the fresh grass and forage they're eating in the garden. With that, you'd have to restrict garden time for a few weeks to see if that causes any improvement.
One other possibility to consider, is due to their time in the garden, they may have picked up intestinal worms that are causing the cecotrope issues. You could ask for a fecal float test done, possibly on both the fecal balls and cecotropes, to see if there are signs of parasite eggs. But sometimes the test can come back negative even though the rabbit does have them. In which case it may be worth asking your vet about trying a 5-10 day course of panacur just in case.
Medirabbit: pinworms
Another very slight possibility, is other internal issues like heart or liver problems. These would be less likely, particularly because of your rabbits young age, but are possibilities, especially the heart issues that can be more common in large breed rabbits. To determine if these are possibilities, more tests would need to be done (eg. blood tests, xrays, ultrasound, etc).
Those would be my suggestions. If it's food related, I would expect some, at least minimal, improvement in a couple weeks once the offending food is removed from the diet, but depending on the severity of the microflora imbalance in the cecum, it could take several weeks after the diet change, for the mushy cecotropes to completely clear up.
If you've treated with panacur, and the diet changes haven't made a difference, you might need to discuss having further tests done with your vet. And yes, I would suggest seeing a very knowledgeable rabbit vet. Not sure who the best is near you. I know Iain Cope at Newmarket Vets4pets, is a highly recommended rabbit vet from what I've read, if that's close enough to you. Or check the rwaf list for other recommendations.
