Like a regular SEO project, we started with fixing the major issues first, which included things like 404s, 301s, unnecessary redirection loops, missing tags, meta etc.
This helped the site to have a better technical score, however, the visibility still remained low.
We then went on and did a complete on page SEO architectural shift on the website, creating different interlinking and on page structure - to make the site look more cohesive in Google's eyes.
We wanted to make sure the crawl depth is <3 for all pages and the bigger pages are passing proper link juice to the neglected pages.
This took a while but once done and indexed, this has done wonders.