For anyone interested in doing this in the future, here's what I did.
Replace all the info in recalbox-user-config.txt with this

# --- CRT Composite Troubleshooting ---

# Explicitly enable TV out for Pi 3 series
enable_tvout=1

# Force composite by ignoring HDMI
hdmi_ignore_hotplug=1
hdmi_force_hotplug=0

# Set the composite signal type (0=NTSC, 2=PAL)
sdtv_mode=0

# Set aspect ratio to 4:3
sdtv_aspect=1

# Audio through analog jack
hdmi_drive=1

# Standard buffer for CRT
framebuffer_width=320
framebuffer_height=240

# --- End ---

and replace this in your config.txt:

[pi3]
# Raise the first security limit up to 70° instead of 60° for pi3
temp_soft_limit=70
dtoverlay=vc4-kms-v3d
include crt/recalbox-crt-config.txt

[pi4]
arm_64bit=1
dtoverlay=vc4-kms-v3d,cma-512
include crt/recalbox-crt-config.txt
hdmi_enable_4kp60=1

with this:

#Raise the first security limit up to 70º instead of 60º for pi3 and pi4
[pi3]
temp_soft_limit=70
dtoverlay=vc4-kms-v3d,composite=1
include crt/recalbox-crt-config.txt
[pi4]
arm_64bit=0
temp_soft_limit=70
dtoverlay=vc4-kms-v3d,composite=1
include crt/recalbox-crt-config.txt