I have a pet peeve: ‘self-publishing’. There is no such thing as self-publishing. Does … not … exist! There is either hobbyist publishing (which doesn’t count as a business activity), publishing with a recognised house (big or small) or publishing independently (indie publishing).
What makes an indie publisher? For an individual it is realising that they are responsible for the production of their work in its entirety, from creating content, to engaging consultants, maintaining quality control, budgeting, negotiating supply chain deals, marketing, marketing, and yes, more marketing.
Indie publishers who happen to also be the author of their work are CEOs of their own publishing house, however small.
Authors who are not thinking like a CEO really need to start now. Get educated. Get networking. And you must read this on the cost of publishing your book.