I tried AoC for the first time last year, and that was pretty much my experience. A week or so of easy problems, then 1 or 2 that were still pretty straightforward but a bit more tedious, then 1 that was a lot more work because you were supposed to derive some of the rules from the example. I don't think it would've been too hard, but like you said, it was starting to feel like a chore at that point, so I stopped.