I could use a little input here.

I have an older site (using Wordpress software) that's been inactive for around 3 years that I'm considering starting up again. The site has roughly 5,000 posts that are, in my opinion, useless. A lot of the posts are poorly written and majority of the content is no longer relevant at this point in time. While these posts still do attract a little traffic, I would like to start fresh and not have the older content have a negative impact on the site, even if only a minor impact.

My question is, what should I do with these posts?

Would it be wise to just straight out delete every single one? There's potentially hundreds of backlinks pointing to these posts on forums and other blogs, links I have no control in removing. Would a large amount of 404 errors like that have a bad impact on me?

Would it be a better idea to file all the older content under a specific tag and/or category and add a nofollow/noindex tag in the robotstxt file telling Google to no longer index those posts?

An insight you could provide me would be greatly appreciated. I don't know much about SEO, but I do know I don't want to make the wrong choice here.

Thanks in advance for your recommendations.