Some learning products are just content with zero container. Books are the limit example. Karpathy's "Let's build GPT" is another.
Most learning products -- and all apps -- live or die by the container they create. (There is no reason to build a learning app other than to build a container. If you feel you have the best content, ship it on a content platform and save yourself a very painful distribution slog.
Duolingo is in the container game. Their container is made of every cheap trick in the book -- notifications, streaks, etc -- because they work. My startup was Hack Reactor, the coding bootcamp, and we did it with pair programming and fixed classroom hours. (We had great content, but our competitors with good containers and bad content did leagues better than vice versa.)
If you're building an app, you're in the container game. You can build a great container with no cheap tricks. I have done so! But you can't build a great learning app with no container, and you can't build a great container if you if you don't want to change your users' patterns of engagement and attention.
So, what is your container? How will you weave a powerful spell that meaningfully transforms the attention and engagement of the app's user? What will cause them to pull up your app again and again, when they would have churned from a simple anki deck or whatnot? Given that you find it distasteful to use the easy levers you mentioned (notifications, "streak" psychology), what alternatives can shift your users' patterns of attention and engagement towards the learning task?
If you have great answers to those questions, great! If you don't want to build a container, build content on a platform with easy distribution. If you want to build a container but you don't want to shape your users' attention or engagement, you are confused.