Some background: I've taken Spanish in school and college, but my receptive language and written comprehension far outweighs my expressive language. I've done some Duolingo, Pimsleur, Say Something In, and Language Transfer. The app is closest to Pimsleur, or other speech/listening spaced repetition audio you may have tried. However, it includes flash cards and targeted practice so you don't have to repeat sections as often if you're not confident. It's also nice that you can see the words on the screen, in case the speaker has an accent you're less familiar with. It also saves your progress so you don't have to question, “where was I last”? for that time you got the stomach bug and couldn't find the motivation for a few days. Duolingo might be the most addicting, but Natulang does have the option to set a reminder to study.
Thank you for such a comprehensive review. While making Natulang, we had to choose between making it addicting or effective, which often conflicts, and we chose the second path. It's a pleasure to read that users appreciate that. Good luck on your learning path.