This may not be revelant, but for debian I have to install the following packages:

sudo apt install libxcb-cursor0 libxcb-icccm4 libxcb-keysyms1

I think for NixOS you need to find missing dependencies by:

ldd venv/lib/python3.11/site-packages/PySide6/Qt/plugins/platforms/libqxcb.so | grep -i "not found"

references: "qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found."