For me personally, the most overwhelming thing in my early days as a backend engineer was dealing with a lot of bugs that my Django apps produced. And fixing those bugs was a really hard process because I didn't understand much of what was going on under the hood. Learning Flask in the very beginning will by no means by easy, but I'm pretty sure that pain is unavoidable regardless of the framework chosen. Choosing Flask at the start will probably postpone the point when a person is ready to ship products, but I believe it will lead to a much better foundational knowledge that will be an advantage in the long run