If it was me, then, I'd be debating either
the microKorg or
the miniBrute, both excellent choices. OR, possibly the new
Volca Keys (which I'm most likely getting); cool little analog synth keys with some fx features, but not optimized for "real keyboard players" (very compact package with keys that are smaller than
microKorg's-- BUT, for your $500, I think you'd be pretty close to being able to get all three Volcas-- three different units, Beats, Bass, and Keys). But if you want the smallest possible "real keyboard" (like, you have some chops on keys and at least average to large-size
fingers), then I'd go microKorg or miniBrute...I absolutely love just about all Korg products, but I might be tempted to go with the Arturia...
GJ