The spine root offset problem you're hitting with MetaHuman is usually because Mixamo normalizes the root to world origin at frame 0, but MetaHuman's root bone sits at hip height. Most people fix it in the retargeting step but I've found it cleaner to bake a root motion offset correction directly in the BVH before it ever touches Unreal — less surprising behavior when the animation system tries to blend.
For custom rigs specifically: if you're using Unreal's IK Retargeter, spend time on the chain mapping for the spine. The default auto-mapping almost always gets the spine chain direction wrong for rigs that use a different number of spine bones than the source. Manually setting FK/IK chain goals for each spine segment takes 20 extra minutes but eliminates the weird torso compression artifacts entirely.