99% of the time flicker is usually caused by Z-buffer fighting as materials fight over who should be rendered first and not always easy to figure out who is doing what.
Since it’s just testing use insane numbers.
Try increasing your shadow map resolution.
Try increasing your light-map resolution.
Check your materials. If using a map that defines displacement (normal maps comes to mind) disconnect it and see if the flicker goes away.
I did a thing for our guys that might explain z-buffer fighting but fair warning it’s wordy.