Our cats are very similar, although mine doesn't have a UTI, we're dealing with a skin tumor with the failing kidneys. Very fussy eater as well and would not eat ANY of the vet recommended diets, even home made food (which even I thought was super gross looking
). But she would eat Fancy Feast (I switched from Friskies), and since she was 15 I figured she'd had a good run and deserved to at least eat what she enjoyed. The vet gave her till spring and then we'd be looking at possibly putting her down, that was a year ago. Her levels have gone up a little but she's doing well.
This past weekend I thought she was slipping on me but it turned out her skin tumor was inflamed, so she got a shot of antibiotics (those are wonderful I have to say) and tonight she was eating like a champ. Over the weekend she had no interest in moist food so I kept offering her dry treats, she does have some fancy feast dry food but she'd only eat a little and the treats have more calories - she's always been too skinny. And really, now that she's 16 and a half, I pretty much give her whatever she'll eat. I've had enough cats that passed away quickly and I didn't get to spoil them because I didn't want them to get any bad habits, so she's pretty much getting away with murder these days
Good luck, hope you're both doing well