My other rabbit, Little Bunny has had sludge for about...well I can't even remember. I think it has been about 6 months now. She has tried the Lasix, which helped Chase with no improvement. She goes back to the vet on Friday and I am going to ask about the catheter flush. I am hoping that is a good option. I can tell you with Chase's surgeries to remove the bladder stones, the surgery was pretty stressful and not something I ever want to go through. I would really push your vet to do everything else before considering that.
With the sub q's, Chase who is about 7lbs got 100-150cc a day and Little Bunny who is about 5lbs got 50-100cc a day. Sub q's never really seemed to make a difference with either of them.
Would your vet consider Lasix with the sub q's? Even a month of Lasix might do the trick. It has been a life saver for me, but more so for Chase. The Lasix really helps her. She has been sludge free for a few months now. However, the Lasix has done nothing for Little Bunny. I would never recommend someone try Lasix unless it was a severe case of sludge, and I think yours is.
Also, if you give veggies, soaking them has really helped me. It gives them more water to help push the sludge through.
Claire, is correct with the incision attracting sludge. The vet said that sludge will sit in that area and will probably never go away, but it is a small amount.