Even not using yocto but buildroot the concepts of what is required to make rpi boot faster are there. Also using Qt application.
https://www.furkantokac.com/rpi3-fast-boot-less-than-2-seconds/

I had been having the same issue except using an iMX8 som. Using eglfs_kms platform plugin and setting QT_QPA_EGLFS_KMS_ATOMIC=1 seems to prevent any intermediate writes to the framebuffer. No more flicker before application startup.