The PGBK cartridge is only printing the top half of each pass.
I have tried
Sample output image of the test print attached (the image is curved because the page was not on a flat surface when taking the photo).
Anyone have any ideas how to remedy? I don't mind buying a new printhead if I know that's the solution (othewise I'd just buy a new printer - probably less expensive).
Based on the steps you have done, I would say there is a issue with the print head. The printer was retired in 2012 so it would cost less to replace the printer compared to trying to get a print head.
You do qualify for the Canon Upgrade program. This is for out of warranty units which have been diagnosed to require service or are incompatible with a new or upgraded system. This program allows you the opportunity to purchase a replacement product discounted from the list price. Free ground shipping is included with your purchase.
If you would like to take advantage of this offer, please call our Sales Department at (866) 443-8002 mon-Sat, 9 AM to 9pm EST. Let them know the Canon Upgrade Program was offered.