How to modify mixamo animations with blender

Im working on a project in UE5 using a mixamo character with its animations.
I want to create a custom animation for the same character while i maintain others mixamo animations, so I exported the same character to blender and created the animation, but when I import the animation back into UE5, it appears deformed.

I guess the issue is related to differences in the bone structure after editing in Blender. Im cracking my head trying to find a solution and i think it shouldnt be that difficult, but im new to all this blender rigging stuff.

How is the way to go in this situation?

Also the animation i want to do is really simple, would i be better trying to learn how to make the animation in unreal instead of blender?

Thanks in advance