"-pix_fmt yuv420p10le" didn't work for me, I get "Incompatible pixel format 'yuv420p10le' for codec 'hevc_videotoolbox', auto-selecting format 'p010le'" which then fails with the same message, "[vost#0:0/hevc_videotoolbox @ 0xXXXXXXXXXXXX] [enc:hevc_videotoolbox @ 0xXXXXXXXXXXXX] Error while opening encoder - maybe incorrect parameters such as bit_rate, rate, width or height."

I don't understand why a standard image size (1920x1080), and I'm assuming a standard pixel depth etc., don't work - I get this for everything I try to transcode with the HW encoder.