@Maxwellsmum our girl has overactive thyroid (hungry/skinny/weird behaviour), an autoimmune condition where she doesn't absorb nutrients from many foods (hungry/skinny/overgrooming), plus 3rd stage renal failure. Untreated she looks a mess. She is on thyroid meds and pred to treat autoimmune disease and with the meds, for all her issues is doing/looking pretty well - gained weight and good quality of life. Pred also helps asthma, arthritis, and appetite (as we all know). Our vet has told us he'll let us know when it is time, but not to count her out any time soon (17.5 now). We adore her and wouldn't let her suffer. Vet expects she'll get cancer at some point and that will be her downfall.
I've heard they do say goodbye though and take themselves off, but as you said he has come back and older cats can get more relaxed/affectionate/sookie too? I know it's expensive, but even if vet says the worst, hopefully you can take him home if he isn't suffering too much? We have thought our girl was done for a few times - once at 16 and then 17.