If the quality of life isn't there, you shouldn't let an animal suffer.
Are you sure she isn't being active when you aren't around? I know someone who keeps a motion camera on their snakes so they can watch them later on instead of interacting when they are quiet.
For curiosity sake, and forgive me if it was mentioned and I skimmed over it ...Do you know what caused the mite infestation after you brought her home? As a blood sucking parasite, they can transmit diseases as well. Especially if they had been in contact with a sick snake before interacting with yours.
As much as we can suggest and as many stories we can share, this is an animal you have had for quite some time. There are definitely changes that happen as they age, eating less, poor body tone and so on..You know what has been 'normal' behavior for this animal. Sure, we suggest some possible alternatives to husbandry or improvements based on updated care guidlines...but that's something to change slowly and not all at once to shock them (like adding enrichment items and changes in decor). It can definitely make it tough to understand the behavior changes and such.
But if this is so out of the ordinary and treatment is no longer an option for you, either relinquishing her to someone who wanted to try or euthanasia would be the two options left.
Very much something that you want to speak to the vet in depth about. Sometimes, it helps to ask them "what would you do if she was your pet? Honestly". Because sometimes, yes... There could be an option they didn't mention due to costs... Or they will give the straight answer of there not being a quality of life they are comfortable with.
Also, the worst thing that could happen is the vet declines to euthanize. If they feel that for some reason that animal can turn around and do well with minimal invasiveness? They can absolutely tell you to go somewhere else.