What is the spare current (after you subtract what’s needed for the OSPi) that I can draw from the +5V regulator LM2596S for my external circuitry?
LM2596S is rated for 3A output, although in practice I don’t recommend drawing more than 1A because 3A is only a theoretical limit, and the circuit has never been tested under 3A output. Just for your reference, RPi normally draws about 250 to 400mA (with WiFi dongle), and the OSPi circuit draws very little, so the overall current draw is well below 500mA.