Hey, just purchased after watching a Quaternius video - hopefully this will ease the pain of trying to fix broken Mixamo animations. Thanks
Hope it's helpful to you. A lot of focus was on compatibility when retargeting in engines, so hopefully you can mix and match whatever animations you need in engine.
If you just need to edit Mixamo animations. They have their own plugin but they don't update it much. You'd likely need to use and older Blender version for compatibility sake.