It is true that forwarded mail has traditionally been an issue. However, ARC (Authenticated Received Chain) was created to address the authentication issues caused by indirect mail flows.
In the Infrastructure Overview dashboard in Everest, you can filter your DMARC reporting to show only forwarded mail or exclude forwarded mail. Use the navigation menu at left to go to Monitoring, then Infrastructure. Select Add Filter at left, choose ARC (Forwarded Mail), and then select whether to show or exclude mail that passed ARC prior to forwarding.
By comparing your overall DMARC reporting with your filtered reports, you can see the impact of moving to a reject policy. Often, the impact is overestimated and the actual impact is negligible or non-existent.