How to set end date relative to start date in date range field
The current widgets in core for the Datetime Range module do not allow this.

You would need to create a custom widget for this.  Off the top of my head, I would create one that extends DateRangeWidgetBase() and mostly mirrors what DateRangeDefaultWidget() does, except to collect a relative period in a plain text field for the end date.  Then, add a massageFormValues() function to turn the relative date into an absolute one, and then call parent::massageFormValues().  You may also be able to do this with a #process callback on the end date element, but I forget which of the two runs first during form processing.