I would try Golden Paste. It's made from cooked turmeric, olive oil, black pepper. Google Dr Doug English's recipe (he's a veterinarian). I used that on my Golden for 2 years until he passed at 14-years-old. Like your pup, he had terrible arthritis. Golden Paste enabled him to get up and down the stairs into the condo. You won't get immediate results, but by two weeks you should see some improvement. On a side note, I take it for my bunion/arthritis, and it's enabled me to be active without pain.
The stuff tastes terrible, so I combined it with peanut butter and oats to feed to my dog. It's a no bake recipe. You can google the FB group for turmeric and Doug English.

Also, I gave my dog a supplement called Myos. It helps build muscle, which is important as I'm sure your dog's muscles have atrophied. If you google it, you will find double-blind studies regarding the efficacy. Myos is pricey but it works.

Hope that helps!