Greetings @ducnn,
Looking at the link you provided, we don’t use GDM on our systems by default. Meaning you’ll probably need to adjust the Plymouth splash screen service. There was another topic about this here: Overlap Docker and Plymouth services boot
Basically you would modify the service so the Plymouth splash screen stays up until your containers are started. Most likely the Weston container will start up first before your Qt app fully starts. You should be able to give Weston a black screen appearance before it transitions to your Qt app.
You may need to tweak some other timings to get the effect desired for your system.
Best Regards,
Jeremias