In order to drive the LEDs without drawing so much, much more than the Arudino I/O pins can handle, I ordered from handy, pre-made driver boards from Amazon (here).
Today I hooked everything up to build one of the “brain” units (battery, board, switches):
The MOS Module seen there allows switching of a larger power supply (5V / up to 1A, from the charger IC) via a smaller signal (I/O pin from the board).
However, while the board is sold as working with a “5V / 3.3V” power supply… it just doesn’t. False advertising, mis-selling or honest mistake, I don’t know. But they use an IRF520 FET, which needs way more than 3.3V at the gate to make any decent amount of current flow.
I did some digging around and found this helpful page, which gave a few ideas. The IRLZ44 looks perfect – so next up I’ll be ordering a load of those.