What on earth do you do for $400k/year? Make moon rockets in your garage or something?

I wish. Sadly that's an example of a prestige job that wouldn't necessarily pay the max. If you're making $400k as an IC w-2 employee you are probably working for one of a few well known companies in a small number of industries like ecommerce, ad-tech, or fin-tech.

Basically I make fastish web servers. Turns out when your company makes billions of dollars small efficiency gains pay off.

Nowadays, this might not be true. 10 years ago, I hired Oracle dbas (not many, mind you) at hourly wages of 300K+ (edit: yes, they worked 40hrs/wk at least)

You can't possibly mean 300,000$ per hour with 300k+..

OP certainly means an hourly wage that added up to 300k per year.

What's funny is that if he did, that would be a low-demand expertise with a lot of willing suppliers so he probably would be making closer to $40k/year.

Are you hiring?