Implementing structured data markup (such as that from schema.org) might seem like a one-time project, but that “set it and forget it” mentality can land you in hot water. You should be monitoring the appearance of your rich snippets on a regular basis to ensure they are pulling in the correct information. As you change the content on your website, this can alter the markup without warning.
Love the five different area of investigation that you went over, great way to analyze and diagnosis the issue. I would also definitely agree doing a rankings comparison between the two time frames, and not only check what your Google ranking is, but also track the search volume for your keywords to see if it has fluctuated or gone down. Google Trends is a great tool for this as well, as one of your keywords that your ranking for may have just lost popularity online.
The thing about SEO in 2018 is that Google changes its algorithms more than once a day! Reports say that the company changes its algorithms up to 600 times a year. While the majority of those updates consist of smaller changes, among them is the occasional, major update like Hummingbird or Panda that can really wreak havoc with your traffic and search rankings.