Here is a suggestion. I am sure some of you thought about this but I did not find anything in the merch space.
Let's think about files (not design, not product). In the "create products" page, when you upload a file (4500x5400 size for example), it populates a number of products at the same time. If you replace this file while in the "create" phase, it again populates the same products with the new file.
Therefore, for validation and review purposes, a bulk submission of a file (e.g., 4500x5400 size) would still enable Amazon to check if the image and the metadata are in synch with the policies while enabling us to have several products submitted at the same time.
This would consume only 1 submission of the x available for the day. Then, another file would consume another slot and so on. So, 1 submission equals several products with the same design file, all with the same image and metadata.