I've worked at an organization with a similar timeframe for some types of data requests (B2B, not GDPR-style ones). There were many parts of the organization which were mismanaged, but that wasn't one of them. That type of data request ("get all my data") involved walking through all the data we had. It wasn't indexed in a way which made it easy to grab.
This was an expensive batched job we ran monthly. We spun up a cluster of cloud machines. A map-reduce style operation would organize the data by customer. We'd ship it off to all the customers who requested it that month.
Adding appropriate indexes or similar would have been man-years of engineering work. This involved, for example, walking through server logs line-by-line and seeing which ones were associated with which customer.
There wasn't a compelling business case to do that. For normal operations, once a month was fine. If a customer had a particular need,, we could hypothetically do a one-off request out-of-line, but customers used the data for types of analytics where a one-month delay wasn't an issue.
I know of other pipelines with similar delays, for example, due to lack of automation. A person runs a task once a month, and automation would cost more than a person.
I won't chalk this up to dark patterns, so much as speeding things up having zero business value to Amazon. I just walked through the process, and at least the first two steps seemed very normal. Amazon sometimes does outrageous things, but here, I saw nothing to get outraged about.