Fair disclosure, I do not currently practice Shotokan, but I do practice Tae Kwon Do which has a front stance and requires 180 degree turns. There may be technical differences that I am unaware of.
Drawing Into Center
Typically, when executing a 180 turn there will be a drawing in to center to recover balance. This drawing in will compensate for any forward weight in a front stance. Yes, it is clumsy, but part of forms training is learning how to execute powerful moves in awkward positions. My suggestion is that you may be missing the drawing in to center prior to the turn, or it may be a technical requirement of your teacher or your style that there be no drawing in to center.
At 17 Seconds in this video, the Shotokan practitioner appears to be demonstrating the drawing into center that I am referencing. Notice how his moving foot passes very close to his standing foot as the foot moving to the front stance passes at a straight diagonal from one front stance to the next.
Zenkutsu Dachi as with TKD front stance should be 70% weighted forward. My rule of thumb is that the knee must not go in front of the toes or behind the ankle.
Pivot on the Ball of the Foot
I re-emphasize that I am not fully aware of the weight placement in Shotokan, but in TKD, the weight is mainly in the balls of the feet most of the time. Some of the awkwardness of spins and turns can be overcome by ensuring that you are properly pivoting on the balls of the feet.
However, in the aforementioned video, the practitioner does seem to be pivoting on the ball of the foot.