This is an amazing answer (if there was an option to mark it as such, I would have). Thank you.
You, sir/madam, are a machine and I'm not surprised this has taken you years. Unfortunately, I don't work in-house for this particular ecommerce site so I only have a few hours a month to work on this. The site's been around for a few years, the physical department store itself over a century, and I've only been working on it for over a year. There's over 10,000 product pages split across hundreds of categories and there are hundreds of separate landing pages based on brand, range, designer, sets, etc (I've been culling a lot of these...).
The vast majority of products contain duplicate descriptions across the whole brand range, so I've mainly been getting rid of those to strengthen the category page so that there's not dozens to hundreds of duplicate paragraphs shared with the category page. But the product pages look so bare with what's left of the description.
I think I'll take a leaf from your book though and go through the most popular categories, aiming for 100 words per product. With smaller ecommerce sites, this would seem obvious to me, but I just wasn't sure whether time could be better spent elsewhere with a larger site.