It's so hard when they aren't doing well. I've been in cat rescue for over 40 years, and the last few years, I've pretty much had a change in mind set. Like you, I've had some negative experiences at emergency vets, and more recently with a regular vet and a cat specialist.
I have a bunch of old kiddos in an enclosed sanctuary on a 3-acre piece of property and I am letting them live out their lives in peace as much as possible. I've decided not to intervene as much as I might have done in the past. Of course, if there's something readily fixable, I will most assuredly do that. With that many years under my belt, there are some things that I feel comfortable even treating myself. Should one be suffering, I will also most assuredly take action.
But I had one recently who was obviously at the end. He was the kind of little soul who I knew would want to go out on his own terms, and I let him go as I felt he wanted to. It was peaceful and loving. No intervention. There wasn't anything to make me believe he was uncomfortable. Yes, I know cats can hide things, but I also knew THIS cat. He felt secure with me holding him and talking to him and keeping him snuggled. It was an extension of the bond that I'd had with him for many years. He wrapped his paw around my finger and we sat together until he was gone.
There are no right answers versus wrong answers. We are mere mortals. Mere humans trying to do the right thing. We do the best we can with what information and resources we have. They know that, and they honor that. So, take a breath, take minute to think about things, and then forgive yourself for whatever it is that you think you need to be forgiven for. Because you haven't done anything wrong. All you've done is commit to love another living creature.
I wish you and your house panther all the very best...