Truth be told I will go on that limb and say the majority of fiction writers are hard-pressed to be supporting themselves solely on their fiction writing. On book sales.
I’m not saying it’s not possible, but, you need to be a constant, dedicated, disciplined and knowledgeable writer. You must work at it. Treat it as the business it is.
But, those advances? What advances? Any given are given against your future royalties. Well, then self-publishing. You get all the profit. And pay for cover art, editing, and still get the smaller percentage of profit after the distributors take their cut.
Why do it? That’s a whole other conversation.
Writing is a business. A personal business. If this is your business – treat it seriously. If you’re not in this business, please take care in your questions. Please don’t ask the monetary questions you would not want to be asked of you.