Hi,
This would need other users to share experience. There are topics about building QT on Jetson Nano:
PySide2 (Qt for python) installation on Jetson Xavier - #5 by Muscle_Oliver
Jetson Nano and Qt5 - #2 by WayneWWW
Pyqt not working on Nano - #4 by coolbot
How to Install PyQt5 Jetson nano - #2 by prince.alondra
There is no topics about cross compiling. Would see if any user have tried it and can share suggestion.