I still have some of the same thoughts you have several years after losing my first two. One of them died from kidney failure too (among his many other neglect related issues). The feeling like you could have done more is pretty normal. 9 years is a really great run and I'd love to have my guys for that long. At that age a system failure of some kind is really not unheard of and I don't know of much to treat kidneys except keeping them well hydrated so they can do their job. The fact that she lived into old age tells volumes about how well you cared for her and what a good life she had. She lived a longer and fuller life than most dragons get and I'm thankful to you for that.