Just start writing. Publish it somewhere, anywhere. GitHub Pages is probably the easiest for you, and may help if Medium leaves a bad taste in your mouth. You can host it from your own domain later with minimal changes.

Why are you dismissing Substack, by the way? There isn't a single platform on earth that will conjure up a readership from nothing, and Substack is no exception. It seems like it would be fine for your purposes, I don't see any reason to reject it either.

My advice: stop procrastinating and start writing. I used to be in the same place as you. GitHub Pages + Jekyll are your friends.