Welcome aboard, Line. I'm also from the New York area - I work in Manhattan and live in Westchester.
You do have the terms a little mixed up. You're a playing card designer - a new playing card designer who hasn't produced a deck yet, but a playing card designer just the same. Playing card artist would also be a suitable term.
A cardist, sometimes called a card flourisher (or in certain circumstances, an eXtreme Card Manipulator, or XCM'er for short), is someone different. Do you remember seeing magicians fooling around with a pack of cards between tricks, doing fancy fans, shuffles, cuts, etc. and making all kinds of decorative patterns with their flourishes? Roughly a decade ago, a handful of people decided that these flourishes were pretty cool on their own and started performing them as a separate and distinct art form from magic, in the process pushing the boundaries of what card flourishes can look like. Cardist is the more popular term for the performer and cardistry is the more popular term for the art form. Among the more prominent performers of this art would be Dan and Dave Buck, De'vo com Schattenreich and Ekaterina Doubrokotova. Many if not most cardists started as magicians, many still perform as magicians, but some perform cardistry exclusively. Many if not most cardists are teenagers or in their early twenties - it requires a fair amount of manual dexterity, something youth often have in abundance! Just as with magic, the majority are males, but there are a number of notable female artists as well.