The idea is great, although I'm a little against the spirit of releasing this as a paid product instead of an Anki plugin/front-end or something. Alas, my biggest feedback is marketing stuff: the english on the landing page is way too broken for a premium paid product; I shouldn't need an account to try a simple app; this is surely not the ONLY (in all caps) app to do "variations". Maybe be more detailed on what the variations are and how it's different from the bajillion free Anki plugins out there.

Also, why SM2 instead of FSRS?