Change the print settings to a higher dpi and/or adjust the density. You might have to fiddle around with some other settings in there to get it how you like it. Generally if the test page prints okay there's something wokny with the print settings for whatever app/driver. Test pages print with preset settings, so they are standardized.

Could be unrelated, and you might just have issues with the printer head, but I've closed a more than a few tickets back in the day for people by adjusting print settings for them (these were brother label printers so YMMV).

Here's a generic KB for doing this on most models of Brother printers:

https://support.brother.ca/app/answers/detail/a_id/83445/~/the-printed-pages-are-faint,-light,-or-faded.-how-do-i-improve-print-quality

Kind of, yes. 603XL has many compatible options, but the 604 of current printers was not available for cheap when I looked a few months ago.

IIRC unless you go ecotank-tier, you are pretty much guaranteed some surprises. Good that they have a "reasonable" range, though.