The difference in adult vs. senior is the drop the protein level. Seniors need the protein. I would personally put the dog on the Complete Health Grain Free Chicken. It's got a decent amount of protein, a lower amount of fat than other regular adult foods...Just change next bag.
If the Dasaquin works, then it's fine. We had an active lab with elbow problems, it presented me with the challenge of finding something that actually worked. It took years, but once I did, I saw results. You can always add in a fish oil pill each meal. I have my 11 y/o girl on Glycoflex 1. But she's not really showing any joint issues at this time.