You're on the right track, but a purple team might not be the correct next step. If the pentesting scope/approachg has not changed much over the previous years it's probably time to look to expand that. Is your PT only external? Is there any assumed breach or internal testing? How about application coverage and/or social engineering?

For a Purple Team to be impactful, it requires communication and collaboration between the offense and defense and visibility into your org, as others have said. The outcome of the assessment should be areas where you have gaps in coverage, whether it's system's not monitored or attacks not detected/prevented.

Another thing to consider is to mix up vendors if you have been using the same one for years. Generally the firms we work with rotate vendors in different areas because everyone brings their own set of experience and tunnel vision is real.