I'm a firm believer that having work-life balance makes every freelancer a better writer. Knowing something about how the world works, whether from a regular, full-time job and/or your family, will increase your empathy and overall emotional intelligence. I've worked as a business communications trainer, coaching Fortune 500 executives, for the last 8 years, and that work has helped broaden my perspective as a business writer (the writing has helped my training work too, by the way). I've also worked in financial services, print journalism, and legal services.
You know your financial needs and needs for work-life balance better than anyone else, and this understanding should help guide how much work you can take on. Yet many writers find themselves with the opposite problem -- not having enough work and thus needing to invest lots of time in marketing.
The freelance writing business is filled with ups and downs, so learning to navigate them effectively is key. Do good work and keep good lines of communication open with clients. Seek to understand the client's business and how you can help solve problems (with good writing). Don't assume good times are here forever, nor bad times.
You also need to be on the lookout, always, for ways to expand your base of opportunities. Freelancers wear two hats, writer and marketer, and need to wear both well. Get your name out there by building your own website and social media platform. Read about trends in the areas you write about and try to take advantage of these trends early. Network face to face with people who can help you, whether with advice or recommendations regarding potential clients.
Don't just market yourself when your pipeline is empty, but make it a lifelong habit, because you'll need it to be as you continue forward. That's true whether you work a 9 to 5 or not.