Hi,
No solution, just a few comments (I do not have this printer model).
If necessary, what happens if you save/convert a document in/to an image format such as JPEG before attempting to print directly from a USB flash drive (or memory card)? Apparently, there are some limitations for PDF files here.
For printing from a computer via USB cable: As you have found out, there are no MP550 Mac drivers from Canon for versions later than OS X 10.11 El Capitan. In case you could borrow an old Mac (OS X 10.11 El Capitan or earlier) computer, that would of course solve the problem. An old Mac running OS X 10.9 Mavericks to OS X 10.11 El Capitan (with the printer connected by USB) could also be used as a networked print server via a third-party app called Printopia, thus allowing printing from iPhones/iPads.
https://www.canon-europe.com/support/consumer/products/printers/pixma/mp-series/pixma-mp550.html?type=drivers&language=EN&os=OS%20X%2010.11%20(El%20Capitan)
There are Canon drivers for up to Windows 10, so if you have access to a PC, that could possibly be another temporary workaround for printing via USB cable.
Regarding Mac (USB) printing with versions macOS 10.12 Sierra and newer, everything would be experimental. The Gutenprint supported printers list shows a Canon MP550 series bjc-MP550-series driver, but it is unclear whether it will work with systems later than macOS 10.14 Mojave.
As always, make sure that you have backup copies of all important files before testing anything.