I run a "family" of five travel sites that are interlinked back and forth (follow) from sidebar, homepage, and within articles.
I've recently relaunched the flagship site with new Wordpress template, fresh content, SSL, validated code, structured data, etc -- it's the one I want to put all focus on going forward.
Its rankings are pitiful. Ten years ago, the site consistently ranked in the top-3 for my search terms. Now, I can barely make third page.
My fear is that the flagship site is being penalized because the other sites in my network have fallen into disrepair. I haven't posted new content in 2+ years on some of them. The templates are archaic and not mobile friendly. They date back to pre-Panda, so many have short posts and thin content. Also, they were created when article directories and hubpages were a "thing" so inevitably there are still those sorts of links pointing at them.
All sites are in Webmaster Tools / Search Console. So Google clearly knows I am the owner of these other sites as well as the flagship site.
The other travel sites are no longer profitable, but they do still receive traffic. I would like to phase them out. What is the most SEO-gentle way to accomplish this? I still have 10 months left on the domain terms. Should I:
1) Ride it out and let domains expire?
2) Take the sites down immediately?
3) Redirect traffic to flagship site via .htaccess?
4) Put index.html page on shutdown sites explaining how site has been retired and provide link to flagship site?
Is it safe to "recycle" some of the good content from these old sites once they are gone by reposting it on the flagship site?
Apologies for the length of my question. I wanted to explain the full scenario.
Thank you for your help!