This seems like a low-effort high-value feature to me. Allow Portfolio Administrators to set system-wide default start and end date custom fields so that each Plan is created with these fields as scheduling field.
Current situation:
Companies might decide not to use the Target Start and Target End dates as the default. But every plan that is created will have the Target Start and Target End date configured for scheduling. Users will typically start using the plan as it was created. And they will be very confused due to the mix of date fields (plans using the Target dates while their Jira Software Board shows custom date fields.
Users will have to change the scheduling configuration of each individual Plan.
Desired situation:
A Portfolio Administrator will set the system wide default scheduling fields. After that each plan that is created will get these dates for scheduling.
Adding my vote as well. We leverage custom fields and set these manually on every plan create.