My daughter's elderly cat was on thyroid medication, and she would get sick and not eat or drink. That, of course, placed her at high risk of dehydration (and more than once the cat was either hydrated by the vet or else my daughter had the little devices at home.
In any case, the vet prescribed an anti-nausea medication and also Pepsid tablets (I don't recall if the cat took only a quarter or half of the Pepsid or anti-nausea; but those were part of regular medications. My daughter used pill pockets to get the cat to eat them, but then we discovered that rolling up the pills/parts of pills in a soft, moist, cat great worked quite well (and less expensively).
If you haven't already done this you may want to ask the vet if either an anti-nausea or Pepsid would be OK. Maybe, since your kitty is still eating, she wouldn't need one or the other of those. I'm not sure which "variety of upset" (nausea, acid, general lack of appetitie), exactly, it was (other than just not eating when she felt too sick to eat) that stopped her from eating. I only know that when she got her anti-nausea and Pepsid she'd eat well (and not vomit).
Neither of those cost a whole lot per bottle (at least in the doses the cat was getting), and the bottles tended to last quite awhile. (My daughter's cat is no longer living, but it was an unrelated medical condition and surgery that was responsible for that - not the thyroid issue. She'd lived with that for a very long time.
You've probably already thought of this, but something else might be getting her a "gentler" food. Having had lots of cats over the years, I, personally, am not a fan of the foods that have "greens" in them. That sounds great to humans, but I've known an awful lot of cats (mostly older ones) who seemed to have "issues" (that required cleaning up :/) from eating food or treats with greens.