Incredibly difficult to know what to do in that position. One of our old cats had a long term health condition and had been very slowly deteriorating for a year or so. The uncertainty was horrible, from both directions, ie did we let her suffer longer than we should or conversely was she still relatively happy when we made the decision. It's harder to judge with cats than dogs as they're far less likely to show they're in pain or discomfort.
Exactly that @gg1 .