I definitely understand that desire for control. What I have found about self-publishing is that growing a following is *extremely* difficult now and you pretty much need a following to sell books. Most social media platforms are in "post-reach" mode, where they're squeezing creators who want to use the platform to get new followers, basically requiring writers like us to pay the platform in order for people to see what we post. This is why TikTok is so big: it's the only platform where creators can reliably reach an audience. I highly encourage you to follow Cory Doctorow here on Medium and read all that you can about his concept of "enshittification." It really opened my eyes to the state of things, and not just social media.
As for Medium and Substack, I think they're currently the best way for writers to get a following, but they're likely to do exactly what all the other platforms have done and go into post-reach mode to leverage their user base. It's just the business model. Justin Cox (he runs the Medium publication Writing Cooperative but is deplatforming to Ghost) is another great follow. He writes a lot about this stuff.
I especially recommend reading these guys if you're thinking of trying to write full-time. It doesn't matter how well you write if no one reads it.
As for not being at the whim of others, I get it but you'll probably find that you can't avoid it. Even here on Medium, stories get "boosted" through some arcane magical ritual that (we're told) involves an actual person reading articles and clicking a button for good ones. Articles that get boosted make about 5 times more money for readers and those that aren't boosted tend not to get read by more than a hundred people. Talk about whim!
But the other side of that whim is that it's awesome when that other person is a good editor. I had the very good fortune to (briefly) work under a Pulitzer Prize winning newspaper editor. He dragged me through the mud several times because of mistakes in my writing, and I learned from it and now I think I'm a better writer and thinker. These editors are the gatekeepers to journals, which I'm increasingly of the opinion are the real gatekeepers to the traditional publishing world (together with simple fame).
