I think what I would do is top it. Continue pruning later. You probably are going to need to get it transplanted into a larger pot in the next year or so. You can probably keep the tree pruned until you can get the tree into its permanent home. Watch it for sun scald. An Avocado is an under-story tree in the jungle. They need indirect sun while they are juvenile's. I wish you luck with the tree. I started out with 2 and one died. As it turned out, I am glad one died since they get so large that I would not have had enough room for both. Avocado's are not easy to grow actually. They need proper care. Fertilizer and irrigation. And, they like a lot of water. I am very proud of my tree in the back yard and it is beginning to bloom this year. Last year my tree did not bloom. Several years the tree bloomed but did not produce fruit. The one year the tree did produce fruit they were very large and very high quality. Very lucky because the tree is seed grown. Many seed grown tree's never produce.