"Is there a way to overlay live workout metrics, such as heart rate or calories burned, onto my boxing or weightlifting videos?"
"Is there a way to overlay live workout metrics, such as heart rate or calories burned, onto my boxing or weightlifting videos?"
Use a video editor, for example: Virb Edit. You need a .fit or .gpx file from the activity whose data you want to overlay on the video.
You will also need information on how to force video rendering in 2k/4k resolution : forums.garmin.com/.../exporting-in-2-7k-or-4k-60fps