I found an easier way of using Mixamo and MetaHumans, that saves having to try and fix offsets with the joints.
1- Export the Meta Human Rig as an FBX
2- remove the extra joints (so it looks more like the mixamo skeleton)
3- Import that skeleton to Mixamo and it will auto rig it. (this step gives animations and skelton that match closer to the original Meta Human, so less mucking around with retargeting offsets)
4- Export Animations from Mixamo
5- Bring into Unreal
6- Retarget as normal (making sure bones are in the right order etc)
As an added step I also used the Live capture with Iphone for the face.
You can see the result here
hope this helps.