The 5000-5500 models squirt 14 picoliter droplets, the photo quality wide formats models of today a minimal 4 picoliter droplet and have at least some diluted grey, cyan and magenta inks to create even more smoothness.
Changing the 5500 to a hexatone B&W printer is still an option but it will not print color then. Still asks for some dedicated skills to get that done but it should not be impossible in my experience with HP printer conversions.
Met vriendelijke groet, Ernst
500+ inkjet paper white spectral plots: OBA content etc.