Well, I would follow the vet’s advice- all of the therapies recommended have demonstrated clinical benefit for arthritis. Adequan and Legend are regular parts of life for many older equines; so are hock injections. These are pretty normal courses of action to help a horse with the beginnings of hock arthritis, which is not an unusual finding for a 15-year-old horse.
Motion is lotion for older animals, humans included, and although yours is by no means senior she is also no longer young. Would more turnout help? Sure, it might. 12 hours turnout is generous in some areas of the country and if you can’t get her out more you acknowledge that you have a limitation there and you’ll have to lean more on other options to better support her body. @AllTheCarrots made a great point about activating the horse’s limbs in the warm-up and I think that you should read that post about four times, go do that, and you’ll probably have a better horse.
You can also talk to your vet or body worker about stretches that you can do on the ground before riding to help the horse start looser. At least 10 minutes of active walking warm-up will prepare the horse’s body for your ride- that’s 10 minutes of the activity AllTheCarrots describes, added to probably about 5 more minutes of walking to get the blood flowing enough to start activating her joints and soft tissues.
In addition to all of this, my horse also responds well to magnetic hock boots in winter- he shows improved range of motion after wearing them for 15 minutes- and you might talk to your vet about Surpass, or OTC diclofenac cream, which is a topical NSAID that can be helpful in de-creaking us. However, the vet’s suggestions are all good starting points with research behind them. Personally, I would start with Adequan or Legend, picking whichever my barn could administer if you don’t give IM or IV shots yourself, and see what you have before going into the joint.