@Shioriiii
Thank You!
My issue is that using AvatarPoseSystem:ExtraBone leads to upload failures (The VRCSDK build was aborted because the VRCSDKPreprocessAvatarCallback 'BuildFrameworkOptimizeHook' reported a failure). I will look into this further on my own. If I manage to resolve it, I will share my solution. Thank you very much for your updates!