It’s too late to lop it off and graft it unless you do the side veneer like Cookie Monster said. If you want to do a cleft draft, you could lop it now (making sure to leave a good number of leaves In the canopy) and hope that it pushes several branches in its final growth flush this year. Then after the first flush next year hardens off, graft into those shoots.
Alternatively, leave it as-is over the winter and then lop it off as low as you want when it starts warming up in spring. Graft it once the first flush hardens off. This would let you get a lower graft and possibly use less scions of that’s a concern for you. You just need one scion to take off on a short stump and you’ll have a full mango bush/tree in a couple years.