Hi,
Since omx plugins are deprecated, please use v4l2 plugins such as nvv4l2h264enc. For 1080p60 we would suggest set bitrate to 14Mbps - 16Mbps. And run in CBR mode and set virtual buffer size. Please take a look at
Random blockiness in the picture RTSP server-client -Jetson TX2 - #5 by DaneLLL