I've let 14 dogs go. The majority were greyhounds. None had osteo. Some had cancer. Some were just old, very old. I *think* I let them go on the right day, except for Pudge. I waited a few days too long with her. And still in her eyes I only saw love and a regret that she couldn't stay longer as she was my velcro mutt. She knew her body was failing her.
I swear there's a look they give. One when they tell you it's OK to make that final vet appointment. The sign is oh so very subtle, or maybe I'm reading my own needs into the moment.
The other look is when they are released from pain by the first injection. The body relaxes. Sometimes they look at me. Never have I felt that look said "why" or "no". In between injections my little galgo Niña, sat partway up and kissed my vet.
I am sorry that you have such difficult decisions to make.