Hi. Firstly,I'm not a vet and this is just my personal experience. Secondly, I am so so sorry this is something you're dealing with. It's absolutely awful. My heart goes out to you.
I was in a very simular situation a few months ago. My beautiful 13 year old boy was not eating, not grooming and hiding away. My usually affectionate and gentle boy was not there. This happened over a matter of days.
I took him to the cat specialist and after tests etc they found large cell lymphoma in his intestine. By this point he could not toilet properly and was losing weight rapidly.
I had 3 choices. Chemo, palliative care or euthanising him. The longer I took to make my decison the further his health declined.
My understanding was that this is not something they usually recover from in such an advanced stage.
I made the choice to have him euthanised as I couldn't bear to put him through more pain when the outcome for him was pretty bleak. From the inital appointment to this point was a week. He was in kitty hospital on a drip with pain medication and a feeding tube as he was not in good shape.
The euthanasia was very difficult for me. It was awful and I miss him every day but I know he is no longer suffering. Following the procedure I had him cremated and am saving for some momento mori jewellery to be made with his ashes. Not for everyone, I understand. But I want him with me every day.
Good luck. It's a very difficult situation to be in.