Chuck Wendig discussed what authors make.
To go from peak to peak, you do what you can do.
What you can do is write the best book you can write.
That is, of course, nowhere near enough to save you or survive — bad books can do well, and good books die on the vine all the fucking time. Luck is a factor. You can lean into luck, but you can’t manufacture it. (Put differently: it’s easier to summon lightning than to create it.)
So, you not only write the best book you can write, but — you write as much as you care to write. How do I do this thing that I do? How do I personally survive the financial aspect of the writing life? I do it by writing a whole goddamn lot. That softens the valleys and lengthens the peaks because I keep a steadily rolling series of advances, royalties, and D&A payouts. Plus, I ameliorate all that with self-publishing money — that money comprises maybe 25% of my total annual income, but it comes faster and with monthly regularity. Ah, but here’s the rub –
Some traditional publishers have non-competes, which makes it harder to publish across multiple publishers and, if they’re being really rough on you, harder to publish self-pub work, too.