First, so glad Abe is feeling better and good job on getting the fountain - my two started drinking more from the fountain than they did from regular bowels.
I can just share my experience with Dante. After our e-vet visit, the e-vet faxed over his findings to my regular vet. The e-vet also prescribed 7 days of Baytril, an antibiotic. I was told to contact my own vet the next day. My vet said to watch urine output and how Dante acted in general, but, absent any problems, to bring him in for another urine sample a week after the Baytril was finished. My e-vet had not been able to get a urine sample.
Ok, Dante was diagnosed with struvite crystals when my vet got the urine sample. My vet recommended Hills CD, and, as Dante has never been a fan of wet food, we got both wet and dry CD. Dry CD a huge hit. Wet chicken is ok but he's not keen on it, the fishy version gave him soft bowel movements. Follow up urine sample at the one month mark showed nice clean urine. Over the following year we went for urine samples about every three months, all of which were nice and clean. At his last senior panel, there was a little blood in the sample, but a sample a month later was clean. Sometimes a little blood in the sample is the result of the test itself (it involved a very large needle).
Did the vet suggest prescription food? Or are you supposed to check in after the Clavamox? FWIW, when my girl Dharma was on Clavamox she also was less energetic and actually didn't eat much - I think it upset her tummy a bit (it's a form of penicillen - I don't do well on that myself!).
In Dante's case, the C/D has worked exactly as it's supposed to for two years - so I don't care if it's made of up old tires and gets the worst reviews from laypeople on the internet in history

- it's worked for us, and my cats seem healthy and happy (I get senior panels twice a year). There are several prescription lines, if your vet suggests going that way - the trick is finding out what your cat will eat. The Hills S/D is sometimes prescribed as an initial diet, depending on the type of crystal, followed by C/D, as C/D is a complete and balanced food for urinary health and long term maintenance. Myself, not tempted to switch foods unless sometime happens healthwise, and will work with my vet at that time, if we need to switch foods for some reason.
Based on my experience, I would suggest you work with your vet on diet, and if they request urine samples, just go with it - it's the only thing that will let you know how the treatment is working.
Oh, FWIW, the Hills C/D is very nutrient dense food - my vet worked with me to find the right portion sizes, as it's easy for cats to gain weight on it if the portions get a little too large. And, I don't find it more expensive than any mid-premium to premium brand. And, of course, finding nice big pee clumps is worth the expense!