I'd strongly recommend making them yourself. Everyone understands these things differently, and ideally your cards will complement the structures already represented in your own mind. Ideally, a card will shine light on one tiny aspect of the model you already have in your mind; if you're using cards shaped around a different model, and you drop them into place around your own model, the lights may be at strange angles or highlighting big complex things.
Can definitely relate, Anki started to make sense after I started creating my own cards. Not batch-creating mind you, but carefully crafting each one, and updating when necessary while reviewing.
I used to think so too. Then I read the research on this, and the evidence suggests the opposite of what you wrote: your time is better spent doing retrieval practice than creating your own cards.
I think the counterargument is more along the lines of: Downloadable decks will have a fairly high percentage of irrelevant cards for you, and will always lack a lot of useful cards for you (the latter is something I'm pretty sure is true).
When I am exposed to some new thing, parts of it will stick immediately, and other parts will need some kind of processing on my part before I internalize it. When you are exposed to the same new thing, those parts that stick and those that need processing will usually differ from mine. Hence it's hard to make a deck that works well for the majority.
