Blender import is easy, check you selected both armature and model.
Check "Selected objects", "export animations", "all actions".
The file is 30mo and export is 11mo, it's too big considering the model polycount.
Your model has one armature per animation while there should be one armature only with all animations.
Perhaps you didn't import well from mixamo to Blender, do you have some mixamo FBX export ?
