If the transaction succeeds they immediately get a thank you email with the recurring terms prominently repeated again. There's also a link to the cancellation button, which is available immediately.

I've accepted this sort of deal as a user and think it's only nefarious if it's not clear or if cancellation is difficult.

Absolutely. A lot of dark patterns are prominent. They prey on an innate human fallibility. In this case, memory.

If you're going to offer a free trial, then you should not need payment information until the end of the trial. You could start sending out reminders to add the payment information towards the end of the trial, instead of an email with a cancellation button, and that would not be a dark pattern. Either practice is a means to the same end, but only the former eliminates the possibility of people paying for what they didn't intend to.