CBD Oil vs Buprenorphine

Non Diabetic Kitty question: Serena is 18 years old in November. Very spry, limber for age. Good appetite. Extremely thin, bony. Fed Friskies, Fancy Feast Pate and Kitten Formula, Dr Elsy dry, chicken liver and gizzard, egg, boiled chicken. Even with thyroid close to normal she can not gain weight! Absolute muscle wasting.

Hyperthyroid on Methimazole.

Recent labs on 8/22: T4 4.2 (high end normal 4.0) BUN 56 (high) Cr 1.6 (normal) Vet gave no recommendations for kidney support.

My concerns are (besides the weight): Is she in pain? I am not sure. She is very ornery with the other cats, hissing, snarling, flipping on her back when they come near. Sits with me but tolerates being held only briefly. She walks OK and jumps on bed, chairs with minimal difficulty. I understand cats can hide pain and disease so not sure how to "read" her. No apparent anxiety. Sleeps like a .....cat!

I am seeing a new Vet this week for followup labs. I want to ask about a long term pain med in case of pain so I would like to go in armed with some concrete info from people on this forum.

I read Bupe not recommended with Methimazole and it should only be used short term. Serena did use it for a few days without any ill effects during a recent bout with bladder infection. I have also read a few people on this forum have used CBD oil. The article referenced below makes it sound promising.

https://boulderholisticvet.com/cannabis-101-cbd-cat/

So.....any suggestions? Outcome of using CBD Oil? Where did you buy it? Any other pain med recommendations?

Thanks!