Backoff in WIP

We are 12.2.8 ebs. Have for a very long time (over 15 years) struggled with replicating a backoff we had from component completion to parent start date as we had in a main frame world. We can achieve this with externally sourced items with Post Processing LT. But never quite was able to get this in WIP. We have added a last operation to represent some period of time but all that did was drive an earlier start date and the completion date of the child was still equal to the start date of the parent.

Thanks in advance if there are any creative solutions