Why to struggle fitting measurement circuits to watch?
Just make usual BLE watch "multimeter look design", with some very basic functions (like mentioned in video voltage detection, maybe digital "mcu/arduino friendly" measurements) and rest of functions take over BLE from other multimeter, like 121GW. (With major problem - you cannot set mode, dial position, over BLE)
Low voltage leads for 3.3V level digital measurements can be added inside, with embedded leads, that you can pull out like james bond garotte