I have admired Miss Buckley since I was a child but since I am closing in on 60, that suggests that, at 75, she is long past the age to play Joanne. (Before anybody says it, I feel the same about Patti LuPone.)
Even more than their age, neither has ever projected much sexuality in their work (that I have seen) and I think you need to believe that Joanne is a very sexually active woman in early middle age). As older actors go, Christine Baranski could probably still pull it off (her portion of Ladies Who Lunch on the virtual Sondheim at 90 concert was stellar).
Jane Krakowski could be good. Victoria Clark, maybe?