Well to adress each in a very brief way:
Beta readers: The best come from those who already read what you write. Particularly those who may have criticized your work in some way.
Proofing: Hire out. Trust me! I've self-edited via grammaryly, text-to-speech, and re-read THREE times and didn't catch everything. Hire someone.
Promoting: It really depends on the genre. These days, you may catch an eventual break doing being dedicated on TikTok with romance. Mostly, you'll probably have to pay for ads if you want a decent income. But it doesn't have to be a HUGE budget, especially to begin with.
Let me know what you're writing and I'd be happy to tell you more.