As a long time SRS user, I couldn't understand the flow of the US capitals example at all. Using Chrome/Linux, I only had one answer to begin with, and that was really confusing. The gradual multiple choice didn't make much sense for me, and it won't make much sense for language. With language, I just want to click on it and see if I was correct with my verbal guess.
I don't like the waiting for ten seconds for a new flashcard at all. Why am I staring at your screen for ten seconds? or a minute! I'm just going to leave and not return.
I personally have no interest in SMS, but others may. What I care about is push notifications and mobile software. (I paid the $$$ for anki, but it doesn't have a way to remind me to go through my cards daily.
Honestly, I think for this kind of startup, you'd be well served to read more of the research of learning, because what I see doesn't match up with what I've personally learned on my journey.