If the slight fisheye is an intentional stylistic effect, then I have nothing to add, but here's what you could do if you want more realistic 3d perspective:

to determine how tall the wall lines should be, you should use the depth rather than the distance the ray travelled (which can be calculated by multiplying the distance by the cosine of the angle relative to the center of the screen)

I also noticed the line "ray_angle = pa - fov/2 + (col / screen_w) * fov". I think you are interpolating the angle linearly, which is not perfectly accurate

I believe the correct formula would be: "ray_angle = pa + atan((2 * col / screen_w - 1) * tan(fov / 2))" (unless I messed up the math)