ffmpeg intermediate transcode for 1920 x 1080 8-bit YUV 4:2:0 @ 25fps source
Are either DNxHD or DNxHR capable of outputing to 4:2:0?

No. Running ffmpeg -h encoder=dnxhd will list encoder properties, including
Supported pixel formats: yuv422p yuv422p10le yuv444p10le gbrp10le


Can DNxHD transcode my source? If yes, how should I ammend the above ffmpeg parameters?

The DNxHD encoder only supports a limited number of preset profiles. Each profile is specified of a frame size, interlacing mode, bitrate and pixel format (which also includes chroma sampling scheme).
FFmpeg will automatically convert the input to a matching pixel format, but the rest has to be manually set.
So, for a 1920x1080 8-bit YUV 4:2:0 @ 25fps source,
ffmpeg.exe  -i &quot;input.mov&quot; -c:v dnxhd -b:v 115M &quot;input.mxf&quot;

Try to encode with a non-matching to prompt an error message with the list of supported profiles.