This has been said before ( https://www.reallusion.com/FeedBackTracker/Issue/Separate-Head-Mesh-From-Body-Mesh-on-Export-for-Morph-Blendshape-Efficiency/1/head ), and upvoted multiple times - but there is no news.

There is a massive problem that blendshapes is on the whole body - when using multiple characters in a scene, the files are enormous in the builds. There should be an option to separate the head mesh from the body on export - and it's quite urgent to make this option. As of now, this is a huge optimization issue, and we might have to change our character pipeline as it's not an option in CC3.