I’ve been having the same issue. I can get composite out on Rasperry Pi OS, but not from OSMC. I tried configs similar to shown here.
I wasn’t going to reply, until I saw the photo, and it looks like you’re trying to set this up on a Nissan Stagea, exactly the same as I’m working on!
So, I was curious if you ever got any further, and what you settled on for your solution?
I did pick up an HDMI to composite, and it seems to work, but would rather just use the composite out directly. However now I’m also bit stuck trying to find a skin for Kodi (or other solution entirely maybe) where the text is readable and and menu usable on the small SD display.