@Emma47 - I don't have advice, just kind words. It's so hard to know when to make the decision. We've had two kidney cats. One we tried too hard with and regretted, and the other (Fog) we're not sure whether we tried hard enough, and also regretted though she went out on a perfect day and we don't regret that. We were adamant with the second that we wanted her to go sooner rather than later and not suffer the way our first one did.
I just don't think there's a perfect answer when they get the way they are.
I found this article helpful and knowing that it was a continuum and they highlight what they consider the four stages of death -
https://www.peacefulpassing.com/knowing-when-its-time/ . They used to have better wording along the lines of there is no wrong time once the animal enters the continuum. Our second cat was stage 2/3 and rallied the day we put her down.
I'm not proposing that it is time, and others have suggested solutions that might work. I did read somewhere that once you start thinking about euthanasia for chronic illnesses there is no wrong time after that because your pet is ill enough that you're considering it an option. You know your cat and your capabilities.
We did a lot of Quality of Life scales for our second cat. Here are some of the ones that we used.
https://www.peacefulpassing.com/knowing-when-its-time/questionnaire.html
https://pawspice.com/quality-of-life-scale.html
https://journeyspet.com/pet-quality-of-life-scale-calculator/ (one thing I liked about this was that it considered the human emotion portion of it, we worried that our anticipatory grief was impacting our cat)
Big hugs - it's such a hard decision.
We are fortunate that we were able to start fostering almost immediately. We view it as Fog's legacy to have helped 15 animals find their perfect forever homes.
I'm sorry for what you're going through. I actually have tears in my eyes as I'm typing this.