I think once you talk to your potential client base, then you may see that a tool for spaced repetition is in far less demand than ready-made card stacks on specific subjects.

If it's at all possible (and it is in very many cases), the vast majority of people would rather pick up an existing stack and start learning instead of spending time on creating it from scratch. Ultimately the goal is to learn and the simpler the solution, the better it is.

That is, your target audience consists of people who are not interested in pre-made stacks or who can't find any on a subject they are interested in. Try and understand who these people are, imagine their use-cases and then see how to give them enough as a hook, but without being obviously useless out of the box.

One of the options is to not limit card counts in free, but rather offer paid features that would actually justify a monthly charge - cloud storage of their stacks, shared multi-device use, ability to share stacks with other people or to use stacks shared by others (! e.g. in school setups). This sort of thing. Monthly fees should be well justified and fair.