What is your issue with Mixamo?
I think it is a great piece of software and provides any animation you would ever need for a game, and you can always customize the animation inside unreal engine…
My issue with Mixamo is I started my project before they dropped support for UE4 and changed the format, but for someone starting from scratch they would have no issue using all the new mixamo .fbx formats.
(My issue is that I was using the ue4 skeleton for all of my mixamo characters and when they changed the mixamo characters skeleton it was no longer compatible with the ue4 skeleton upon import (without going into a 3d modeling software and changing all the bone names, but someone is helping me out with that already - and that is a problem particular to my design choices - as I mentioned if you start a new project from scratch ontop of the mixamo skeleton you will be fine[and don’t need to have the UE4 skeleton as mixamo has every animation “you would ever need” for a game…]).
But you are right about the GAP, if someone were to provide something like FUSE does but with full support for UE4 I would buy it without blinking.