A lot of the times I talk to the student before they buy their session on Discord. By that time, I know their goals, since they expressed them to me. I also let them fill out a deeper Google forms questionaire to support my coaching approach and build a data base on the student. For them to feel the need to express themselves again with the mandatory question is not optimal here.

Also, when a existing student buys sessions to extend time with me or to continue a topic at another date, we both know what it is about.

I would propose a skip button for the mandatory question, that reads along the lines of "skip that question, talked to coach".