Hmm, the command seems to give ok output, but VNC still ends up using a low resolution.
I’ve found this thread: Set VNC resolution? - Raspberry Pi Forums
It suggests commenting out ‘dtoverlay=vc4-fkms-v3d’ in [pi4] section (add ‘#’ at the start of the line), as well as uncommenting and changing these lines:
framebuffer_width=1920
framebuffer_height=1080
None of those changes were necessary on the Pi4 I’m using though (and I did try your config directly in place over my one), but modifying those lines may help in your case.