The Qt5 print system is lacking the 'advanced configuration' tab (to choose tray, toner save, etc.). This was Qt bug 54464 (https://bugreports.qt.io/browse/QTBUG-54464). This problem was a showstopper for a lot of people wrt qt5 use. Patches have been made upstream to solve this issue. Those patches should be integrated into the current ebuilds (5.9.4).
*** This bug has been marked as a duplicate of bug 617460 ***