quickest way to improve website seo?

fix your broken internal links too. run a crawl with any site crawler, filter for 404s, point the links at the right pages. an afternoon on a small site.

nothing is truly quick, but the fastest wins I know of: open Search Console, find pages with lots of impressions and low clicks, and rewrite their titles to be clearer and more clickable. then check the pages report for anything important that isn't indexed. after that, link to your best pages from your other pages. none of that needs new content.