Downsizing 2160 ProRes 4444 XQ to 1080 - Best option?

dgwvideo: "...if you select the clip in the timeline and look at "spatial conform" in the video inspector..."

FWIW - In my quick test yesterday, I did download the sample clip and try that. Fit, Fill, and None all yield the same result. The clip is a film transfer and black edges are baked into the image. So frame size is 1920x1080. It's just that actual image content within that frame is somewhat smaller. It's certainly worth trying again, just to make sure, but slight scaling still seems like the best solution.

I already tried all three fit options, all the same result.

Just to add one more option. You can use Compressor to fix the issue. Simply set the cropping settings to match your Clean Aperture offset (16 left, 16 right), if you like. The encoded file won't have the black edges. But when you do this Compressor is scaling. That will be the same as if you scaled in FCP, since they both use the same video processing.