My mom has welcomed my role as caregiver and advocate. I know that might not be true for every caregiver. But if you talk about it, I'm confident you can reach an understanding. Perhaps your role as advocate will be limited to advising your loved one when they need to take a different or stronger approach with healthcare personnel. Eventually, as their health fails, you may find this changes and you truly do become the advocate.
But whichever way it goes is OK, as long as your loved one gets the care he or she needs and is able to live comfortably for whatever time is left.