Publish what you'd want to find
Search for the post you wanted to read on a topic.
If it doesn't exist, write it.
This is the simplest test for what to publish. You had a problem. You looked. You couldn't find a clean answer. You worked it out yourself. Now write the post you wished you'd found.
The post is automatically useful, because it's the post you needed. There's at least one reader: past-you. Almost certainly there are others.
Most writing advice is about audience research, keyword optimization, niche selection. This shortcuts all of it. You are a sample of one. If you needed something and couldn't find it, others probably did too. Write what you wished existed.
Three patterns to look for:
The blog post you'd want to send to someone in your situation but can't find. Write it. Now you can.
The Twitter thread you'd write if you weren't worried about being too long. Write it as a post.
The "I don't know" you've said three times this month. The third time you say it on the same topic, the post has written itself. Just transcribe what you'd want to know.
This works because the curse of expertise is also the cure. Once you're inside a topic, you forget what was confusing on the way in. The post you write now, while the confusion is fresh, is more useful than the post you'd write in five years when you've forgotten.
Write what you wished existed.
That's the whole prompt. Most of writing advice is downstream of this.