I've got an email newsletter that 874 people pay me $5/m to read (The Sizzle - https://thesizzle.com.au). I set it up outside of Substack a few years ago, then migrated when Substack launched with the hope their platform would bring in new readers and make my life easier.
This turned out to be a massive waste of over a year, as not only did the newsletter stop growing, it lost subscribers. There was no platform effect by being on Substack, so I left it, setup my homebrew solution (where all the bits, like billing, email sending, customer info are interchangeable) and growth has resumed and I'm doing better than ever - all without giving Substack a 10% cut and further locking myself into their ecosystem.
Here's a blog post explaining in more detail why I chose to remove myself from Substack if anyone is interested: https://blog.decryption.net.au/t/why-i-use-a-mishmash-of-ser...