1. GSC: find keywords with the highest clicks and impressions. In both cases, we were average position 2-3.
2. Google the keyword and open all 10 pages in separate tabs. Note the headings (I use SEO Minion to scrape them).
3. Dump them all into a spreadsheet, filter by H2 and H3.
4. Now compare them to the headings you already have and select 3-5 topics that you haven't covered (check the volume and/or use intuition).
5. Rephrase those headings and write 200 words for each of those topics, I make them H2s.
I even stuck these new sections at the bottom of the article and it still worked.