Taboola (like many websites) would benefit from iterative SEO, meaning its team should make distinct revisions to the SEO page titles, content headers, and other text to see whether rankings improve without hurting their top positions. But they shouldn’t make too many changes at one time or they won’t know what worked and could disrupt rankings. They could monitor rankings after testing keyword-placement scenarios. (Most website pages are indexed within days or a couple weeks.)
QUOTE: “I think there is probably a misunderstanding that there’s this one site-wide number that Google keeps for all websites and that’s not the case.  We look at lots of different factors and there’s not just this one site-wide quality score that we look at. So we try to look at a variety of different signals that come together, some of them are per page, some of them are more per site, but it’s not the case where there’s one number and it comes from these five pages on your website.” John Mueller, Google
While SEOs can provide clients with valuable services, some unethical SEOs have given the industry a black eye by using overly aggressive marketing efforts and attempting to manipulate search engine results in unfair ways. Practices that violate our guidelines may result in a negative adjustment of your site's presence in Google, or even the removal of your site from our index.
QUOTE: “Supplementary Content contributes to a good user experience on the page, but does not directly help the page achieve its purpose. SC is created by Webmasters and is an important part of the user experience. One common type of SC is navigation links which allow users to visit other parts of the website. Note that in some cases, content behind tabs may be considered part of the SC of the page.” Google Search Quality Evaluator Guidelines 2017
The audit is for all pages and not only one. What happens in the majority of the cases is that pages / posts have similarities so you can group them together. For example the pages of a website may be ok but the blog post pages may be missing titles. It’s a lot of work especially for a 500 pages website but you can start from the most important pages first and work your way to the rest