Slightly off-topic, but something I wonder about (I apologize for my ignorance): why aren't there any 3.5" SSDs? I've seen a bunch of 2.5" & PCIe SSDs, but never 3.5" SSDs. Obviously, they'd be expensive, but I wonder how much more storage 3.5" SSDs could hold.
Its not physical size that is the issue. You'll find even in a 2.5" SSD most of the space is empty. Power usage is a large factor. 500GB+ SSDs tend to peak above 15W so they wouldn't be good for non-powered TB2/TB3.
Another factors are cost, density, heat. You could just stick your 2.5" SSD into a 3.5" adapter.