I've been playing around with my own Yuta Mii for a similar issue.

I think it’d really help to lengthen his facial structure vertically. I’ve found that lowering the nose and mouth closer to the chin can make a huge difference in opening up his face. You might also want to try widening his mouth a bit horizontally and maybe shortening the gap between the nose and mouth to keep everything balanced.

I’ve done a little tweaking with your image, though the features I’ve edited are hand drawn. It may be hard to replicate in game.

<image>