Our Luna, had her heat about two months before we scheduled her spay..we all thought she was done, her breast tissue was not swollen anymore (she did have some milk too, by the way),and her genitals were down in size too..still, when the Vet opened her up for the spay she was in full swing on the inside..
I would wait as long as I could, it would make for an easier spay, I would think.
Of course Luna, being a Min. Schnauzer/Yorkie mix, is little in size, so maybe it was harder because of that.
I have never had a girl dog before in our household, that have all been boys, and I have never had such a pathetic looking dog in my care after a neuter…pi--ed off maybe, but not weak and not even walking for two days..it was rough..
I wish you best of luck, it will be done soon...