I've always believed that a Good SEO audit requires at least one page to be given a true manually reviewed style audit. I'm a huge fan of automation and site spider tools for a whole website, but definitely, it starts with wrapping my head around what needs to be done on a given page.
Probably with a bit of irony, our own website is seriously overdue for an SEO audit, as we've been neglecting it for having been so dang busy in recent memory. Here's a great example of even our own SEO content page being seriously due for some search engine optimization over at https://www.cayk.ca/search-engine-optimization/ but I still consider our own site to be constantly improving regardless. haha