This lens is 3.4 kgs. It is heavier than the Canon 500mm f4 L IS II USM which is around 3.2kgs but lighter than the Canon 600mm f4 L II USM lens (3.9kgs). Do you directly mount it on the monopod or are you using a ball head? A decent ball head +monopod might be better than handholding if you are indeed shooting throughout the day.

These days I don't shoot stills much with my Canon 400mm f2.8 L IS USM lens. While handholding I normally wait till I expect the desired action is taking place and then quickly lift, aim, fire and then lower it. You can't keep on holding the lens. Just look at the way shooters lift the gun, aim and fire at the clay disk thrown in the double trap shooting event.

If you are primarily using this lens with a 1.4x TC, then I don't see this lens instead of a Canon 100-400.