A huge factor is the value you bring and the impact you make. If it’s significant, your career will have momentum and you’ll be able to call more of the shots and enjoy the freedoms you speak of. But if what you bring is simply labor and time then you’re just a part time employee.
I, myself, do work for free. But I do it under certain conditions. Factors include if the gig is juicy and short term enough that I can leverage for other things, if it gives me ability to show off skills that I know I have but aren’t as prevalent in my resume and if the client is influential. On that last one, you should probe. Check out their LinkedIn contacts and ask for specific introductions and endorsements if you deliver. Don’t leave it open ended. Otherwise, find a nice way to ask them to dimensionalize how they are connected and what they’d be willing to do if you perform. Get it on paper.