Just took some time to test out and it’s working good for me.
what I just did:
download from mixamo, import to blender with default setting,
in edit mode create a root, make it parent of Hip bone
in pose mode copy hip location keyframes to root, delete hip location keyframes
in object mode apply rotation
and export with add leaf bone disabled
import to unreal with default setting
if everything is set, you’ll see root’s red line getting longer, and tick enable root motion the anim will come back to in-place.
these are basically standard operations i found out, I think you’d better try default skeleton and mesh from mixamo first in case they have conflicts.