Oh you poor love. It sounds such a familiar situation to our ears. Remember - we had nearly 3 weeks on Septrin (which I
think is the same thing - or at least very similar) and then had to have another three weeks to knock out our UTI. If she was well on the sulfatrim and peeing freely might it be worth arranging with the vet to go back onto it to see if it can make a difference again? When she was comfortable on the sulfatrim did she need any painkillers at all? Or did you notice any occasional blood in pee or weight loss? If she is inflamed with a UTI (with blood in pee) she might dripping, in some discomfort, and struggling to get a good flow going. This itself can lead to urine concentrating in the bladder which increases the risk of sludge... but I bet I don't need to tell you that.
It's interesting you mentioned pH as this is something we are wrestling with at the moment with George (who, post-surgery, has some sludge and faintly squeaky wees but is doing OK and has even gained a little weight). George and my girls are on the same diet, George also drinks a ton more water, and yet George is the one with sludge and who has formed 2 big stones. He is 6, but the others are somewhere between 4 and 5 now. BUT his pee is easily pH 9, rather than the pH 8 of the girls. Piggies are supposed to be around 8, maybe a little over, but not 9. And my spider sense is telling me that could be the root of his problem, with the calcium grit (quite likely calcium carbonate) precipitating out of solution as the alkalinity increases. However, a bladder full or pee is not the same as a beaker in the laboratory so who knows how these thing interact.
My vet was concerned about infection - because infection of the urine with certain types of bacteria can indeed raise the pH because they break down some of the urine for nutrients and the products are more alkaline. But George now doesn't show any of the classic signs of infection - particularly as he's not getting worse (touch wood). So we tried cranberry for 5 or 6 days to see if we could flush what may have been sludge and drop that pH to 8. I was giving George 5ml of cranberry juice every morning on one go (followed by a ml or two of water to rinse the mouth). Cranberry juice is surprisingly acidic and gram for gram I'd worked out that this was like a person drinking a pint of it. A sort of cranberry flush! At first he was quite keen but at the end he wasn't into it and just left his mouth open till it mainly rolled back out again. He's an old hand with syringes
It didn't make any noticeable difference to his urine pH at all. In truth I'd have been surprised because I bet it's been tried a lot in the past. The downsides to this are that the acidic juice may well, if used long term, cause sores in the soft tissues of the mouth and b*gger up the digestion. The body as a rule is brilliant at keeping all it's systems the same so an influx of fruity acids might just have prompted a counter-measure of buffering something-or-other to be produced in order to neutralise it. It won't be that simple for George... his kidneys are as old as the rest of him. But it might just be that your little lady's infection has returned and you just need a longer course of sulfatrim to tackle it. Was she eating OK on this? Did you have to support feed? Or could you just dose her and let her get on with things herself. x