Let's face it: Source's movement is clunky. While players are the most agile refridgerator boxes in any game that I've seen, they're a long ways away from being organic. In a game about chaining stunts together, the "mobile cardboard box" system is especially obvious. I have no idea how much you can change under source's limitations, but you should try to get a few of these in when you overhaul.
1. Smaller, more descreet collision box: Source's player bounds are way too big. Try reducing the size, or better yet, change it to a cylinder instead of a rectangular prism.
2. Automatic movement transitions: Make it so that when you run into a wall, instead of just stopping, you start to run up it. After a few feet, you would fall back down unless you did a wall flip. Let people queue upcoming stunts while current ones are underway. Try to make mantling visibly grab edges, Contagion style.
3. Movement assist: brush entity that dives are attracted to. Slides can just use traces since it's 2d.
4. Get rid of awkward moves: If I slide into a wall, I should stand up quickly so I can wall flip, instead of just turning to the side and slowing down.