I am amazed that she has lived three years longer than they said she would! God bless her, that's wonderful!!! Collies are my breed, they are the only breed I will ever have so I have a special soft spot for them.
You know your girl better than anyone. If she is still interested in things and appears to enjoy herself then I say let her be. I don't know why sub-q would be painful, it's a simple procedure. So simple, in fact, you could do it at home to save her the stress of having to go to the vet all the time. The vet can provide you with IV bags and lines and show you how to slip the needle under the skin. A small bore needle wouldn't hurt at all and, while it would take longer to get the fluids in there than a large bore needle you could just sit with her on the couch and pet her for an hour while you administer the fluids.
Do a Google search for canine renal diets and home-cook for her. You might have to tweak some of the recipes to pacify the liver issues but it can be done.
The bottom line is you know your dog. It's about quality of life, not quantity. When she stops enjoying being with you, you will know it. You will see it in her eyes and there will be no mistaking it. In the meantime, love her and enjoy her. Cessation of eating will be your first sign. She won't want to do the things she always does. But don't live just waiting for that moment. Enjoy your girl while she's here and let her enjoy you.
Ghilly
If you go to Groups on the side of the page, click on that and you will see Chronic Kudney Failure in dogs. Click on that. You will find a lot of good information there.