I have a hobby blog that doesn't have to do with coding, but I post on substack. I enjoy the features on substack, but I would advise that you set up a custom domain if you post to substack.
To build an identity that's easier for your readers to remember and to start getting "credit" for SEO even if you're not actively optimizing keywords, etc. Why let substack reap all of the benefits?
I've been writing small Swift/iOS related newsletter on my free time but I am still using substack subdomain.