I took a look with the ListDLLs tool but couldn't actually find the plugin being used by the 32-bit version, maybe some other DLL is handling that? Anyway, my problem is now solved so I'll mark this topic as solved.
You can set the LD_LIBRARY_PATH environment variable before starting your application. Otherwise, if memory serves well, you need to configure the rpath when building your application. See QMAKE_RPATHDIR.
@Proton-Phoenix I no longer have access to the Zebra I was using in the past (I changed the company since then).
Zebra printers are a story on their own to troubleshoot anyway...
General approach I used:
make sure the printer is configured properly in the system, with the default page size set to the target page size.
query the QPrinterInfo instance for the default page size.
use that data with QPdfWriter to render whatever text + barcode I need.
remember DPI settings, either set what the manual for the printer says (ideal) or 300 (this didn't end well for Dymo printers I used for a while, needed exact DPI of 204 ;) ).
print and enjoy.
In general getting this to work might be lots of trial and error, depending on the brand and drivers. But once it's done right it is literally zero maintenance for years.
The model should provide this information for you. Create a role in the model which will return the correct value for any given cell. On QML side, just access that role and display it directly. No need for any get() method (which, by the way, is called data() https://doc.qt.io/qt-5/qabstractitemmodel.html#data, not get()).