After looking everywhere for a solution with no success I have to ask here:
I can’t get Dorico to automatically shorten the last bar of a piece when I enter a pickup bar. To manually shorten the last bar by changing the time signature and hide the time signature symbol doesn’t feel right.
Select the final beat rest, open the Bars popover with Shift+B, type final, and hit Enter. If you wind up with an empty bar at the end after this, do Shift+B again, type trim, and hit Enter.