"Technical skills are no doubt in very high demand right now. If an experienced engineer can make over $500K/yr plus benefits working 40 hours a week at a large company, there should be an opportunity to spend a few extra hours on a side gig to increase that number by a few percent."

First off, I think that that's extremely rare at that level with those hours.

Second, why would one want to work for a couple extra percent? If the comp is second to some passion, then sure.