Diagnose before you touch anything
Know which problem is worth fixing before you spend a month on the wrong one.
Export ninety days of queries from Search Console, crawl the site, and mark pages with impressions but no clicks, plus pages nothing links to.
A blocker list of three to seven items, each with a URL and the number it affects.
Write the list on one page. If it does not fit on one page, you have not chosen yet.