I've done a few pieces of expert witness work over the 2-3 years (on deceptive design / dark patterns).

Although expert witness hourly rates may look high, it really depends on how many hours you bill in a given period. If you're an expert in something very niche, there might not be enough cases in a given year to give you a meaningful income.

Don't quit your tech job yet. Expert witness work is usually only lucrative if your expertise is in high demand, like personal injury or medical malpractice.