I think that sex reassignment surgery is one of the biggest decisions anyone can make, and I’d really recommend you cum before important consultations and choices, just because you don’t want your denial affected brain biasing your decisions. Same for any big decisions really. Always a good plan to cum before them and clear your head.
In terms of when else, personally I’d have a final cum before the surgery, as a kind of goodbye, but I’m soppy like that. Also, you’re on at least three months no touch after it and you’ll absolutely want to make sure everything’s working, so definitely come afterwards too.
Good luck with it all!!!