When constructing the IKRig in Retargeting animations in UE5 with IKRig and Full Body IK setup [ Uefy v2.5 ] - YouTube I skipped creating chains for IK bones, same for the IKRetargeter in the next video. This is why your ik_foot_r etc are locked in place.
You need to create chains for those bones in the IKRig and then link them together in the IKRetargeter. You can use the IKRig file for the original SK_Mannequin as reference if needed.
All the required ik bones are present in the uefy export with correct hierarchy and names but I have never used the foot_ik controls in any meaningful way. I got foot placement working using the main foot bones so never needed the ik bones.