OK - after reading many posts, including this excellent comment I was able to set up Clean HDMI output AND record to the SD card at the same time.
From the previous comment 2 years ago:
First make sure you're set to shoot 4k.
Next in the menu, Red Tab 8 > HDMI display > Monitor Only
Keep in mind that you will lose ability to record internally when outputting clean HDMI.
Add: Check your menu Yellow Tab 4 > HDMI resolution > Auto
Now, set the camera to video; connect the HDMI connector and launch OBS.
You will see the camera LCD display on the computer monitor in OBS.
If there are overlays; tap 'Info' until you get a clean screen.
Now touch the video record button on the top, and click the 'Record' button in OBS. You're recording 4K to the card and 4K to the HDMI output.
Demo of it working: https://youtu.be/OpQra1vVLJg
