> It was also my impression (from the Anki community) that spaced-repetition is far more effective when you create the cards yourself. A common "beginner mistake" is to download a pre-made deck and start memorizing from scratch.

Varies between people and subject matter I think.

All of my Anki usage thus far has been for language learning, and for this use case pre-made decks have been better than self-made for the simple reason that these decks are higher quality than I have patience to build myself. I do still plan to build my own decks for vocab beyond core 2-3k but for getting off to a running start I think premade is just fine.