Vonnegut had to convince at least two gatekeepers: his own agent and the editor at Colliers.
I spent a large chunk of time a few years back looking into the questions OP is asking, and the ultimate truth I came to is this: whether you're self publishing or going the traditional route, you're going to need some established gatekeepers to support you if you're going to make it.
In 1949, those gatekeepers were traditional publishers. In 2021, we still have traditional publishers, but we also have content curation algorithms, social media influencers, podcast hosts, and platforms like Substack and Patreon. If you can get any of them to put resources into promoting you, you'll have a real opportunity of making it - that is, if what you're offering is any good.
If you can't or don't want to get the attention of those gatekeepers, it doesn't matter how good your content is, no one will ever find you.