If eating isn’t the only measure of quality of life, what else should you be monitoring?
You want to look at the whole animal.
Are they comfortable at rest? Can they move around without significant difficulty? Are they sleeping well? Are they interested in you and their surroundings?
Can they stay clean? Are they breathing comfortably? And are they still able to enjoy some of the things they normally love?
Quality of life is a pattern, not a single checkbox.