How do you approach optimising a large site with thousands of pages?
An SEO audit of the site is the best approach here. Using an SEO tool like Screaming Frog or Majestic will show you the link profile, ranking, keyword density, site speed, top pages and more. This will give you a real list of tasks to prioritise and work through one-by-one. Many SEO tools also give you a dashboard to show your progress on areas like link building and page rank.