Hello,
I’m not sure how much help I can be, as I’m no expert, but maybe I can offer some advice. I think archiving your old posts that are no longer relevant to your new site focus would be a great idea, it would be a shame to lose all of that content. The following is a link to a page from the WordPress Codex: https://codex.www.ads-software.com/Creating_an_Archive_Index
This takes you through the steps to create a single page with links to all of your older, archived posts. This would make it easy for you and your site visitors to access them, as you said there is still some info in there that may be helpful to some people. I hope this is somewhat helpful for you!
Brynne
