My Raspberry Pi screen resolution being stuck at 800×600 with VNC access to a headless Raspberry PI with Raspberry Pi OS. I found that if I comment out the lines towards the end (as shown below) I was able to achieve a 1920×1080 resolution with my remote headless connection.
[pi4]
# Enable DRM VC4 V3D driver on top of the dispmanx display stack
#dtoverlay=vc4-fkms-v3d
#max_framebuffers=2
[all]
#dtoverlay=vc4-fkms-v3d