./ffmpeg -hwaccel_output_format vaapi -hwaccel vaapi -i ~/New_hdd/Swimming\ with\ orca\ in\ New\ Zealand-JQ3mDXF3bcE.mkv -c:a copy -c:s copy -c:v h264_vaapi -f mp4 /dev/null -loglevel verbose

main detail missing was "-hwaccel_output_format vaapi".

with it I can transcode h264->h264 FHD at 42 fps and h264->hevc at 90 fps (know feature of Polaris based encoder at least under Linux).