Designjet Z6 color shift - prints no longer match

[kb428]

My Designjet Z6 24" is no longer printing the same colors, despite using the same paper, files, and settings. Attempting to reprint files created several months ago results in shifted colors, especially cyan/blue, which is now printing lighter/less saturated but with no evidence of streaking or lines. It's possible the colors were drifting slightly already, but there was a noticeable change that started between two prints done on the same day within an hour or two. 

I have tried:

-printing from Adobe Illustrator, Adobe Acrobat, and directly via USB drive

-turning off and back on color calibration in the printer settings

-replacing the ink cartridges, shaking and reseating the ink cartridges, and running the printer's full calibration multiple times (before and after replacing ink)

-uninstalling and reinstalling the printer drivers (btw the latest drivers do not play well with Illustrator, but that's a different problem) and restoring the ICC profile for the paper I'm using (HP Superheavyweight Plus Matte)

-replacing the paper roll with a spare roll of the same type

-replacing one print head (Magenta/Yellow) which has claimed to be "Beyond life expectation" since before the issue, but I noticed no visible quality problems (the printheads are original to the printer, bought in 5/2022, and have run about 600 mL of ink each). The cyan/red and black printheads are both out of warranty but the "health gauge" says "Printhead is within its life expectation."

The printer stays on constantly (except when someone unwittingly unplugs it). It's not used every day but at least a couple times a week. I am at my wit's end and have wasted too much time, paper, and ink trying to figure out the issue. It's printing fairly consistently with these new colors, but they no longer match anything printed before and look less punchy (not to mention branding no longer matches). Suggestions?