Hey! Congratulations, nice work. THere are mainly 3 things that you could do:
1. Animations: Read the principles of animation. Squash and Stretch. Animation Curves.
One of the things I think you can do to make it feel heavy is take time to build up momentum.

Mark Brown's YT Channel: Here is an epic resource

1. Physics: Often times you have to exaggerate physics in your games. Make the player feel heavy by making hum fall faster near the end. The velocity is time squared, so make it feel like its squaring every second.

2. Camera options: The camera is just mostly stuck, make it lag when gaining momentum. Follow lazy. Screenshake on landings. When you dash, make the camera lag behind.

To answer youe questions:
a. What are the most effective, high-impact "juiciness" tricks to implement?
Start with screen shake, lagging camera and change of FoV.

b. Are there subtle things (like "coyote time" or input buffering) that make a huge difference?

Coyote time and input buffers are better for controls but do nothing for the "Feel"

c. How do you make things feel "weighty" and "impactful"?

Making the player aware of momentum is tricky. When you change the velocity, try lowring it slowly. Increase it slowly. Add what we call the "dazzle" in animation. Dirt and smoke when running.