This may not apply to your dog at all, but a few years back we thought our dog had the start of HD, but x-rays showed his hips were fine, instead it was discospondylosis. MSM did not help. We had him on Medicam, but then recently started him on an Omega 3 fatty acid oil (he was also showing signs of senility), and the oil help his mobility so much we drastically cut back the medicam and he is doing better than before, even through winter.
Apparently the Omega-3 oil can help replenish joint oil.
It isn't all that expensive, and has other health benefits so it may be worth a try.
Sorry to hear that your dog is not well. I had a brittany who lived with HD for over five years with very little medical treatment. We got a corgi puppy who I think added a good three years to her life as she helped to keep her more active. The corgi also served as the brittany's ears as she had gone deaf but she could always hear the sound of the fridge opening!
My SIL's husky GSD etc cross was in his teens and started to go downhill. I think it took about 6 months for the dysplasia to become really apparent - the leg became totally useless. He already showed a lot of the symptoms mentioned here beforehand and had quit eating entirely for a while - my MIL invested a lot of time cooking him rice etc., but finally with the dangling leg she (MIL) had to admit it was time (I think the neighbor lady suggested it and it came easier from her, I'm not certain), SIL had suggested it a while back.
His illnesses lasted about a year and a half, or perhaps two if you start from when he quit going upstairs.
He never cried or seemed actively in pain, he just had been a rowdy, almost obnoxious, dog that was now very quiet and subdued.
It's a tough call, best wishes to you.