In the UK/EU market, Epson does not maintain current drivers for this device on their site for High Sierra, or Mojave. Instead, the suggest one get the drivers from Apple, which means downloading/installing the July 6, 2017 Epson Printer Drivers v3.3 for OS X package. There is a DX8400 driver in that package, but based on the date, it might support Mojave, or might not, despite Epson's claim.
The first thing that I would try is to Reset your Print System, and when you add back the DX8400, it may update the driver without the first paragraph update process. See if this regains your ability to select photo quality from the printer specific print panel entity.
If the second paragraph does not help, then download and install the Epson Printer Drivers v3.3 for OS X package, and install it. Then repeat paragraph 2.