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Hello, here is our story. Our niche is mental health (psychology, psychotherapy e.t.c). Our portal has thousand of genuine articles, news section about mental health, researches, job findings for specialists, a specialized bookstore only with psychology books,  the best forum in country, we thousands of active members and selfhelp topics etc. In our country (non english), our portal has been established in 2003. Since then, for more than 15 years, we were no 1 in our country, meaning that we had the best brand name, hundreds of external authors writing unique content for our portal and hundreds of no1 keywords in google search results. Actually, we had according to webmaster tools, more than 1.000 keywords, in 1 and 2 position.  (we were ranking no1 in all the best keywords). Before  2 years, we purchased the best domain in our niche.  I ll use the below example (of course, domains are not the real ones):
We had: e-pizza.com and now we have: pizza.com
We did the appropriate redirects but from day one, we had around 20-30% drop in search engines. After 6 months -which is something that google officialy mentions, we lost all "credits from the old domain.. .and at that point, we had another 20-30% drop in search results. Further more, in any google core update, we were keep dropping. Especially in last May (coronovirus update), we had another huge drop. We do follow seo guides, we have a dedicated server, good load speed, well structured data, amp, a great presence in social media, with more than 130.000 followers, etc. According to our investigation, we came to one only conclusion: that our forum, kills our seo (of course, noone in our team can guarantee that this is the actual reason of the uge drop in may-in coronovirus google core update). We believe that the forum kills our seo, because it produces low quality posts by members. For example, psychopharmacology in a very active sections and we believe, google is very "sensitive" in these kind of posts and information. So here is the question: although the forum is very very active, with thousands of new topics and posts every month, we are thinking of moving it to a subdomain, from the subfolder that now is.
This will help our domain authority to increase from 38 that is stuck 2 years now, to larger scales. We believe that althougth this forum gave a great boost to the portal, in the past 10-15 years, it somehow makes a negative impact now. If I could give more spesific details, I d say this: in all seo tools we run, the best kewwords bringing visitors to us, arent anymore, psychology and psychotherapy and mental health and this kind of top-keywords, but are mostly the ones from the forum, like: I want to proceed with a suicide, I m taking efexor or xanax and they have side effects, why i gain wieght with the antidepressants I get etc. 1. Moving our forum to subdomain, will be some kind of pain, since it is a large community, with thousands of backlinks that we somehow must handle in a proper way, also with a mobile application, things that will have to change and probably have some kind of negative impact. Would that be according to your knowledge a correct move and our E-A-T will benefit for google, or since  google will know that the subdomain is still part of the same website/portal, it will handle it somehow, the same way as it does now? I have read hundreds of articles about forum in subdomains or in subfolders, but none of them covers a case stydy like ours, since most articles are talking about new forums and what is the best way to handle them and where is the best place to create them (in subfolder of subdomain) when from scratch. Looking forward to your answers.