Teams Planner Duplication

So unfortunately Planner doesnt have a native "duplicate entire plan" feature that also copies the linked SharePoint files. What most people end up doing is using Power Automate to clone the plan structure, but you still have to deal with the file links manually which is annoying. 

For onboarding specifically tho, we switched to using Teamflect inside Teams. We initially used it just for the surveys but we found we could automate onboarding tasks in it too. We just create onboarding task groups for each department and they are assigned automatically. We used it side-by-side with planner for a while and eventually gave up on planner.

If you really want to stick with Planner, the Power Automate route is probably your best bet. There are some decent templates in the connector library for cloning plans.