You need to call another enginer this guy sounds like a parts changer, I was in that phase but never works properly, keeping on buying expensive parts wont do it, my monet is on the high voltage.
Sounds like a problem with the High Voltage Power Supply, if the mirrors are cleaned, its clear that this engineer is a noob, it cant be the imaging drum, its more a voltage issue either with the scanner unit or the high voltage power supply or even the desnity being set too low, cold resets do nothing, expect bring it back to factory settings however that would help, have you tried doing a engine test?
Yes in theory it can be a toner cartridge / imaging unit like if the drum is worn and lost its orignal ablity to accept beams from the scanner unit, yet the reason why I say it can be the high voltage the drum gets a charge in the EP process a uniform charge the beam from the laser does the exposure stage and the toner is attracted to those parts of the drum.
then the toner is transfered onto the paper, if the high voltage is impaired with any of these the print quality is faint or if theres too much voltage the printout is too dak, the engineer should have this basic knowlege of the EP process.
I have a OEM toner, in all my main laser printers because refills can cause faint prints or leaks I had a remanf toner that turned out to be a refill the density was impaired, and install a HP OEM toner in this case a LaserJet 4300, full darkness was restored.
If you increase the desnisty settings does it get slighty darker?