The clarity, the sharpness, the pop - everything was improved. Good glass is a bigger difference than the body.
A $300 lens is objectively better than a $5 smartphone lens.
A $2000 lens may be objectively better than a $300 but it depends what you're standing in front of and you.
The Nikkor Z 28mm f/2.8 is my favourite so far and it wasn't exactly expensive.
The priority order for things is of course:
1. What the photographer is standing in front of
2. The photographer
3. The lens
4. The camera
On the flip side, my favorite macro was a Sigma 105mm prime. Tack-sharp, and cost well under $1000. Of course, I’ve never shot with the equivalent Canon L (which isn’t quite the same at 100mm, but close).
But, at least today, you still get enhanced features on the more expensive zoom lens, such as wider aperture, and a constant maximum aperture across the entire zoom range. Neither of those things necessarily yields a superior photograph -- you don't need f2.8 across the whole zoom range if you're taking pictures at f6 -- but they can be very helpful. If they're worth paying for depends on one's personal needs, desires, and budget.
Not sure where you are getting the $5 figure from. In any case, smartphone lenses are manufactured in vastly higher quantities than lenses for interchangeable lens cameras, so it doesn't make sense to compare the per unit cost. Modern smartphone lenses are miracles of optical engineering. See e.g. https://news.ycombinator.com/item?id=30557578 The cost of the R&D that's gone into enabling their design and manufacture probably couldn't have been recuperated if they were being used only in cameras.
-- more control of depth of field / shallower DoF ability?
-- faster shutter speeds?
-- less chromatic aberrations?
It doesn't intuitively feel like sharpness should be a factor -- even cheap kit lenses usually get that right.
The chromatic aberration is an important but subtle effect. Remember that lenses are multiple pieces of glass, and every interface diffracts the wavelengths of light like a prism. One of the considerations in lens design is converging all those different wavelengths of light in the same place. Not just at one point, but at every point across the image plane.
Poor lenses might do the well in an area. Good lenses do it everywhere.
The static aperture also helps tremendously of course, yes - nice bokeh with a tight zoom means you can easily get candid portraits that look great from anywhere in the room.
L glass is also a very interesting used market - those things basically don't lose value IME.
Other than the actual business side of things, pesky details like getting clients. And the massive stress of shooting a wedding. I was happy to do it gratis for family, but I don’t think I’d want to deal with paying clients.