Hi! At the moment I suggest to going on with point (1): open the modified character skeleton (after run “Retarget Mixamo Skeleton Asset”), open the “Animation” tab, select an animation, then in the viewport “Show > Retarget Base pose”. Finally: “Create Asset” > “Create animation” > “Current Pose”. You should end with a 1 frame animation of the retargeting pose that you can export to FBX.