Hi Jeff,
This is a tough one. Very sorry to hear about your business losses.
As I'm sure you know, several the recent "core algorithm" updates from Google have focused on site quality. Via their Quality Rater program, they ask human reviewers, with specific guidelines, to dig not only into the content of the site but the background of the owners/writers. They test new substantial algo changes with this group before they update results for all users, with a specific focus on "Your Money Or Your Life" (YMYL) content, or pages that deal with serious and potentially life-altering topics.
A site offering health information and supplement advice, and also selling those supplements, is in the crosshairs of this kind of review.
This is just my take on it, but I expect your losses are largely due to the perceived brand trustworthiness. I'd consider toning down your on-site product promo imagery and ensuring trust-building elements (badges, ratings, testimonials, any kind of accreditations you have) are clearly visible above-the-fold. I'd also recommend building a more clear/clinical layout and typographical treatment for your advice content (blog posts, articles, etc). You might also want to consider limiting the array of supplements you promote and sell, staying away from the controversial and potentially dangerous.
I also, unfortunately, would not expect immediate results from this. These core algorithm updates come several times a year, but but I worked with an auto parts retailer who lost 30% of their organic traffic+revenue overnight in the "Phantom III" update (which seemed to be a general "quality" update similar to recent core algo updates) - they had some UX issues, content that seemed there just for SEO, etc. About a year after their big drop, they made a big push to improve UX/quality and add trust-building elements to their pages, and six months after this design/UX overhaul, they regained all of their traffic in the "Phantom V" update.
I suspect there is nothing technically broken with your site and that duplicate content and similar are not holding you back much - but that quality raters preferred search results with other sites for the keywords you've been ranking for.
First impressions of the brand, quality/trustworthiness of content, etc have big impact here - but these reviewers are also instructed to verify that the owners/publishers of the site are accredited and trustworthy as per other online sources:
"Many websites are eager to tell users how great they are. Some webmasters have read these rating guidelines and write 'reviews' on various review websites. But for Page Quality rating, you must also look for outside, independent reputation information about the website. When the website says one thing about itself, but reputable external sources disagree with what the website says, trust the external sources."
Not to suggest you have scam/similar accusations showing up online, but it's something additional I'd want to look into.
Best of Luck,
Mike