Good luck with this, Koikatsu body models and textures definitely weren't made with this in mind, let alone clothing lol.
Are you playing with the posing at all in Studio or are you just relying on existing idle animations? Some quick adjustments on hand placement with IK / FK would help avoid clipping on the cuffs at least, although you may have to make the hands grip the jacket instead of resting like that to get anything that looks natural.