I want to hide the tray icon of my qt5 script in python to appear in the expanded box, (as the "Always Hidden" setting usually would in KDE5), rather than directly on the taskbar.

QSystemTrayIcon.hide() would hide it entirely out of view. In the tray settings I can't change visibility either, probably because it's not a permanent entry, the dropdown is greyed out.
You can't do it without implementing a KDE plasma applet(plasmoid). For details, see Plasmoids, Notification Area (Systemtray), and Plasma PulseAudio Applet.