So sorry about your dog. But honestly, this isn't a decision that should be made online. Your vet knows so much more about your beagle and her medical history--he'd be able to give you much better information. Most of the symptoms you mention could be related to the Cushing's--perhaps your vet can suggest a new medicine regimen to help manage it better? At the very least, your dog should probably be seen by the vet again if she hasn't since the new symptoms appeared.
If it helps at all, in the end, we've always tried to make our decision based on what's right for the dog. If your dog still has happy times and an appetite, she may not be ready to go, yet. Our biggest worry is always that we let it go too long because we can't bear the thought of losing our baby. But only you can weigh everything in the balance and decide if the good times trump the bad. It's a very personal decision. No one knows your dog better than you do--especially not people on the internet who don't know your dog at all.
It sounds like you've given her a long life and lots of love. Sometimes, letting them go can be the most loving thing you do, but trying to get the timing of it right is always so heartbreaking. Make your decision out of love for your dog and you can't really go wrong. It's impossible to know when the 'best time' is. You can only do the best you can do. No one will fault you for it.
Best of luck, and please let us know how you get on!