Back pain, OA lower back - how much lbs/gms can be safely removed?

I am 46, heavy dense breasts. Have considered breast reduction for past years but was scared. Now my lower back has severe compression and developed OA. Every pound I remove represents a better quality of life for me (on top of the relief of neck, shoulder and back pain). How much can you safely remove? I don’t mind how small I go, but want to remove the most weight possible to save my back. Can you go from G to B for example? I swim for my back health. How soon post op can you swim?