I would hold off on repotting it since you want to move it outdoors, which would be around the middle of May. As long as you provide it with water, it will be okay in the 6" pot. I don't recommend topping it to force growth. Once you plant it outside, you can top it back to induce lateral branching. One final note: when it does produce, the fruits will not be true-to-type Fuji. The seed will contain the genetics of whatever varieties were involved in the pollination when that apple was produced.