I had a blog (still do, though I publish extremely infrequently), but the value of Twitter to me is the discussions, where I can ask about things and learn new things from people who reply to them. What's the equivalent for that in this system?

Now, if you said "get a personal domain and set up ActivityPub," then sure.

The other approach I've liked here is the "planet" model - I read https://planet.debian.org/ fairly frequently, and occasionally one blogger publishes something and a couple of people follow up a day or two later "This is a response to so-and-so's post, here are my thoughts," which is always fun. But it wouldn't be fun in a newsletter model - Twitter makes it so that you don't see replies from someone you follow to someone you don't, because those replies will generally lack context. I wouldn't want to spam everyone in my newsletter with "These are some questions I have for some other person whose newsletter you don't read."