In order to make serious money, you have to own something - real estate, stocks, a business, intellectual property. Things that pay you without you having to work more.

Real estate can be good. Bootstrap a duplex or triplex and get some passive rental income flowing. Build from there. Obviously investments can also be a good route.

One of my best side gigs was intellectual property-based. I wrote technical books on a popular subject and the royalty income was significant. It also made me seem like the "expert" (I wasn't) and that got me more work in my field. With a PhD, you absolutely have the ability to do intellectual property work (patents, books, speaking, etc). Might be an avenue for you.