Nowadays there are many options of flashcards apps for learning foreign languages. Some of the ones I´ve used for TOPIK are LingoDeer and Duolinguo. They both offer interesting sources of vocabulary and grammar, but as far as I know, except for maybe LingoDeer, which mentions the test at their levels, they are not really focused on TOPIK vocabulary.
I would strongly recommend using vocabulary books like this one and the more traditional Anki if you are an independent learner.
- It is multi-platform, so it runs and syncs your progress on Windows, Android, iOS, from the web browser etc;
- Traditional SRS algorithm, based on SuperMemo SM2;
- Versatile import/export options;
- Costumizable flashcard layout, check this;
- It is general use and free (except the iOS app, which is a different project and it is paid) and opensource, and you can copy text from your card and search for vocabulary on your favorite dictionary.
- Many plug-ins, like the heatmap and many others
- If you add many new cards, it can be really hard to do all the future reviews, because Anki focuses a lot on repetitions. Also, there is no simple option to set some learning conditions, like considering weekends as non-studying days. So expect a lot of card reviews from one week to another if you study many new cards on a given day.
- Anki 2.1 is based on a different ecosystem than Anki 2.0. So older plug-ins may not work on the latest version.
- As for Korean, I guess there might not be many pre-made decks as compared to Japanese for example.
My suggestion would be using Anki as a review tool for the new words you learn from TOPIK vocabulary books or from pre-made decks.