In case you have issues with IK bone retargeting, I would highly recommend starting to watch my series.
ALS Secrets - UE5 Manny Integration + Git Workflow
Constraining bones is covered in this video, #9.0 Constraint Bones - UE5 Manny Integration - ALS Secrets
#9.0 Constraint Bones - UE5 Manny Integration - ALS Secrets - YouTube. At the time of writing this video is in sneak preview. 