Curious, have you ever talked to a physical therapist? I've learned a lot of the maladies people live with are correctable, even sometimes things that seem permanent. Many running overuse injuries, for example, boil down to muscle weakness, muscle imbalance, or bad posture.
Without seeing a therapist, try doing this every day or every few days to elongate your 'posterior chain' and help your posture reset (which will help your knees, ankles, elbows operate under less duress): https://www.youtube.com/watch?v=4BOTvaRaDjI
Why would you blow all your salary? On something that is uncertain to work?
As a DIYer for 8 years 'ish, mudding drywall is an art. Where it might take me 4 or 5 (or 6) to get a good finish on joints or corners, it might take a pro 1 or 2 coats and very minimal if not none sanding in between. Primer and 2 coats of paint won't fill in imperfections greater then an 1/16th in diameter so if you have a lot of imperfections, you'll see it in the finished product.
Let's not even discuss mudding ceilings. The amount of upper body strength needed is amazing, let alone good balance on a ladder.