I've been a professional fiction writer since 2002. Here's what I've learned.
This list assumes your books are good enough. You'll know when you know.
1. Write when you can. Finish what you start. Self-publish what you finish. Repeat.
2. You don't need gatekeepers.
3. The only thing you should pay for is cover art (and maybe formatting if you haven't learned how to do that yourself.) People do judge a book by its cover.
4. The only advertising that ever worked for me is BookBub, and that doesn't work as well as it used to. Your best advertising is writing a good book.
5. Kindle Unlimited earns more than going wide on all platforms. Make sure your Amazon listing sings.
6. Your online presence will not help sell books. Social media doesn't boost sales.
7. Don't read your reviews, or worry about reviews at all.
8. Periodic sales and discounts and freebies increase units sold and income generated.
9. Be polite, gracious, humble, and thankful.
10. Don't stress over sales. Just keep improving your craft.
11. Never give up.