fkms is "fake kms mode", in this mode the kernel selects the screen mode(s) for the screen(s) but the videocore firmware manages the video output path. Using the open source vc4-fkms-v3d driver listed below instead is recommended. `dtoverlay=vc4-fkms-v3d` causes blank screen when enabling 4K@60hz #1392. But definitely there should be option to disable this overlay. But still figuring out how to do it. The Raspberry Pi 4 can be used without monitor in headless mode.