Use a wish

All this time, Boruto still did have the wishing stone. So he used a wish.

Sarada wasn't exactly built how he wanted. She was still under developed. After all, they were the same age. So he used a wish to change that.

Suddenly Sarada had aged multiple years, her body filling out more. She now stood before him a few years older, her breasts still slightly smaller than her mother's, but much better now.

Given his wish, there were stipulations to this. Effectively she would switch back to her 'regular' form most of the time, but when he was going to be playing with her, she would take on this form.