Gosh, she's coping with a lot, isn't she... Sending her so many vibes

I'm glad she's seemed happier since you started treatment for the UTI. I can see why you're concerned about her weight loss, though, and why you're worried you might be fighting a losing battle, and I definitely understand why you wouldn't want to put her through major surgery at this point.
When Sophie had bladder sludge and UTIs I mostly focused on increasing her water intake rather than reducing calcium. With Lilac having arthritis like Sophie did, I'm guessing that might be contributing more to whatever's going on with her bladder than getting too much calcium, as she'll be moving less and might not be emptying her bladder completely if she can't pee in the right posture, leaving the crystals (and urine) to stay in her bladder more than they should. And I suspect most of the calcium will be in Lilacs dry food, which she'll need if she's already lost so much weight, of course... The amounts of calcium in most other things won't be very significant compared to pellets. It might be worth looking into dry food with less calcium, though. My vet put Sophie on a kind of FibaFirst made for bunnies with bladder problems, I can't remember the name... Urinary Something.