This problem could also be caused by a broken power steering pressure sensor wire. Check for a loose green wire near a sensor on the power steering rack. It often breaks over time and causes a fluctuating idle like you described. When operating normally, the sensor increases engine RPM slightly to increase power steering pressure when the driver is turning the steering wheel with the engine at idle, such as when parking.

The replacement tailgate struts from Honda come with a dab of grease in the socket at each end of the strut. Check aftermarket struts for grease and add some yourself if there’s none.

Also, while the old strut is removed, check that the mounting ball bolts are tight. Two of mine were loose.