17 year old with cushing's : when is it time to start considering?

Hello there, I have a 17 year old dachshund, pretty husky, he's a strong old man. So far, he has not lost control of any bladder or bowel movements. However, this last weekend has been a scary one. About 4 nights ago, he took a big fall, and since then it's been more of a struggle to walk. Took him to the vet immediately to make sure nothing was broken, and they asked if he had ever been tested for Cushing's. After doing research every day since then, my dogs matches every single symptom to a T, except for being incontinent. He can walk, and today has significantly been better than the past 2, but no matter how I try to control his food and water intake, his stomach still swells like a balloon ready to pop. His nose is leaking, sometimes grayish pink, sometimes not. His eyes do the same thing. He is still alert, he is still eager and excited for food and when I come to him. Still as bossy as ever. From the timeline, he's had Cushing's at least 3 years, that's when his tail started going bald, and has since spread to his neck, and his appetite for food and water became voraciously uncontrollable. I've been preparing myself for him to go for years since my other dachshund passed 4 years ago at 15 from bladder stones, and he's only 2 years younger than her. Her death was sudden, there were no signs leading up to it, and I've never had a dog this old, or ever had one with Cushing's before either. What signs should I be looking for when his time is here, or when I should make the decision to end his suffering?