I am not sure about the best practice between Unreal and unity but I may suggest an alternative.
First import the Unity model to unreal , then retarget the animations to the new model and export it . then directly import the animations to unity. Theorically i might work :)
Viewing post in Unreal Engine Sex Animation Starter Pack comments
Hello , thank you so much! I really glad you like the animations.
The fbx has root bone and optimised for unrealengine by using control rig of unreal engine. I haven't tested with Unity.
About the commissions , i would love to make but currently i am working on a game and it consumes all the time i have :(
Please let me know if i can help you about the Unity pipeline.
Best Regards