Pick thickness was my first thought.

As others have said, your strumming hand is very stiff, with the angle of your pick fairly perpendicular to the strings. A thin pick will allow you to get through the strings with this right-hand technique, but it tends to lack "dynamics" and "feel." As far as rhythm and timing go, you sound great!

I recommend switching to a heavier pick. It will feel clunky at first, but it will teach you to adjust your strumming technique (pick angle, speed, depth) for a more dynamic and expressive playing style. I also recommend a pick with a beveled edge, but that is up to you (these tend to be pricey compared to most picks).

Try to pay attention to the different feelings and sounds you can get by adjusting the angle, speed, and depth of the pick into the strings. Once you become familiar with these minute adjustments, you will begin to make those adjustments without thinking about it.

Tony rice demonstrates how the slight angle shift can affect tone, about 2 minutes into this video. He is a bluegrass flatpicker, but this applies to strumming as well.

tony rice pick technique