Unless the start-ups you target have cash, current income for your efforts is not to be expected making you an investor. The odds of startup success are so low, that even where you able to make good judgment calls you would have to make 20 such investments and have at least 2 of them pay off at ten times your for cash hourly rate to breakeven and wait between a half decade to a decade to actually have cash.
Hope this helps you think it through
It's just a matter of aggressively filtering your prospects. Anyone can start a startup… even people who don't have the money to pay your consulting fees. You want to make sure you're not spending your valuable time on those who can't.
Perhaps only focussing on marketing may be a tad too niche. We have been providing consulting to startups through Startup Manufactory since 2013 and it has proven to be profitable. It also depends on the size of the ecosystem you're operating in, of course.
When I started my consulting business back in 2012, I tried to work with startups for the first few years. I met many good people, and they all badly needed and appreciated my help. But ultimately I had to stop working with startups because they simply have no money to pay for outside help. It's sad because the need is great. Now, there are ways to do this if you get creative and also have a diversified portfolio. But it will slow your growth rate, and limit your income some. If you're interested in some lessons learned and advice, let's talk further.