So found a similar issue posted on LibreELEC forums
add “composite=1” to the kms dtoverlay in config.txt
dtoverlay=vc4-kms-v3d,cma-512,composite=1
enable tvout in config-user.txt
enable_tvout=1
set your desired mode in cmdline.txt (on the same line after “quiet”). eg for PAL;
video=Composite-1:720x576@50ie
or for NTSC;
video=Composite-1:720x480@60ie
Played around with this and output does work but playback is still quite bad and even stops completely on a basic 720p VP9 video