I began blogging about a year ago on Medium, so hopefully I can answer your question(s).
Regarding which blogging platform to use, if your posts are going to be simple (not talking about the content. Are you going to need Jupyter support? How about LaTeX? Etc.), then I’d tilt towards Medium because it’s awfully straightforward to use and like you mentioned grants you more exposure.
Otherwise, fastpages is your best choice given that it integrates effortlessly with Jupyter Notebook & its various functionalities (LaTeX, collapsible cells, and so on) and it’s also easier to use your own domain.
In short, if you’re seeking something more flexible, then fastpages but if your posts are going to be mainly words with maybe some code or math notations here and there, Medium is easier to use and gives you more followings.
By the way, if there’s one tip with respect to content I could give my one-year-ago self, it’d be know your audience. Are your readers going to be top-tier researchers? If so, there’s no reason for you to waste your time and explain what a fully-connected layer is. But if they are beginners with no knowledge of machine learning, it’d nice not to dive into complicated concepts right off the bat and explain everything in detail.
Do let me know if I wasn’t clear enough!