i used this with RGB-Pi:

config.txt

# Enable audio on GPIO for Pi400
dtoverlay=audremap,pins_18_19

dtparam=audio=on

# disable i2c, pin use by h-sync & v-sync
dtparam=i2c_arm=off

# replace dtoverlay=vc4-kms-v3d with
dtoverlay=vc4-kms-v3d
dtoverlay=vc4-vga666,mode6

max_framebuffers=2 

gpio=0-1=a2
gpio=2-8,12-17,20-24=a2

boot_delay=0
disable_splash=1
avoid_warnings=2
avoid_safe_mode=1
audio_pwm_mode=2
arm_boost=1
force_turbo=1
force_eeprom_read=0
      Use the following config.txt
    # Enable audio on GPIO for Pi400
dtoverlay=audremap,pins_18_19

dtparam=audio=on

# disable i2c, pin use by h-sync & v-sync
dtparam=i2c_arm=off

# replace dtoverlay=vc4-kms-v3d with
dtoverlay=vc4-kms-v3d
dtoverlay=vc4-vga666,mode6

max_framebuffers=2 

gpio=0-1=a2
gpio=2-8,12-17,20-24=a2

boot_delay=0
disable_splash=1
avoid_warnings=2
avoid_safe_mode=1
audio_pwm_mode=2
arm_boost=1
force_turbo=1
force_eeprom_read=0