I agree with your approach. I feel though that the angle to take would be something like mongo... where I do not need to worry about a schema/migrations etc... and rather focus on the API logic itself.
I like the fact I can whip something up quickly with MongoDB, get everything working and then once my schemas and services operating thereupon have stabilised then I can start looking at swapping my implementations out for a SQL-based back-end if I think it's worthwhile.