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
What would be the purpose of/reason for moving back to a different url? If its been a few years, I’d leave it alone unless you watched everything decline since moving to the main url. Moving the forum to a new url now would probably be a bit chaotic, not only for your main url but for the forum itself…. Only reason I could imagine myself moving the forum in this scenario would be if all those links were really awful and unrelated to the url it currently sits on…
Use common sense – Google is a search engine – it is looking for pages to give searchers results, 90% of its users are looking for information. Google itself WANTS the organic results full of information. Almost all websites will link to relevant information content so content-rich websites get a lot of links – especially quality links. Google ranks websites with a lot of links (especially quality links) at the top of its search engines so the obvious thing you need to do is ADD A LOT of INFORMATIVE CONTENT TO YOUR WEBSITE.
Structured data21 is code that you can add to your sites' pages to describe your content to search engines, so they can better understand what's on your pages. Search engines can use this understanding to display your content in useful (and eye-catching!) ways in search results. That, in turn, can help you attract just the right kind of customers for your business.
QUOTE: “The preferred domain is the one that you would liked used to index your site’s pages (sometimes this is referred to as the canonical domain). Links may point to your site using both the www and non-www versions of the URL (for instance, http://www.example.com and http://example.com). The preferred domain is the version that you want used for your site in the search results.” Google, 2018
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