Although the format menu has a sub menu 'custom', this seems to be a further list of set formats.

Is there any way I can set a custom format myself? I am filming in 6K with GH5 and want to keep the same proportions when editing, 4992 x 3744.

Is it possible?
I can edit 6K in Blender, but I don't find the process nearly as intuitive as Kdenlive.

That means you work with 6K anamorphic, 4,992 x 3,744, 10-bit resolution video mode.

- You have to change the project settings as customer to 4992 x 3744.
- You have to change the FFMPEG render settings as well.

The problem would be the 10bit. As far as I know MLT works with 8bit and some effects doesn’t work. More details see here: viewtopic.php?f=270&t=155634

Yes, found the option, an icon on the Project Settings screen, right hand edge just below the heading "Select the profile (preset) of the project."
Have set profiles for 4992 * 3744 and 1440 * 1080.
Rendering on 4 cores much faster than I expected.
One remaining minor problem, after rendering the render completes, but leaves a message "remaining time 00:00:00" that prevents the next render starting.

Not anormorphic mode - using the 6K photo burst mode which actually saves the 4992 * 3744 video at 30 frames per second plus sound, and runs continuously until the cards are full. Using a mains powered supply. Video quality is superb! Just ignore the GH5 options to extract images - the video is what I want.
Looks as though Kdenlive will process it quite happily.