Open Google Search Console and look at the queries report for the last 28 days. Sort by impressions. You will probably find searches you show up for and never wrote about.
That list is better than any brainstorm. People typed those words, and Google already thinks your site is related.
The ones worth writing for
- Queries where your average position is 11 to 20. You are on page two. One good page can move you onto page one.
- Queries with impressions and almost no clicks. People see you and do not choose you, usually because no page answers the question directly.
- Questions phrased the way a buyer talks, not the way your homepage talks.
Track a few, not all of them
Pick the five searches that matter most and watch them week to week. Average position moves around; a trend over four weeks means more than any single day.