Bottom line - an electric aircraft is always going to need knife-edge efficiency. No power will be diverted for anything other than "go" and "payload" - so no pressurization. Thus the practical limit on altitude will be 12500 feet or so, regardless of the physics at higher altitudes. I'm sure there will be some "built-for-purpose" World-altitude-record-for-electric-aircraft contenders, with engineering devoted soley to lifting 1 person, 1 oxygen bottle, and x-many kilos of batteries to 40/50/60,000 feet. But in that case the optimum altitude will be "beats previous record before the O2 runs out"
. U2 aerodynamics would seem to rule in that case.
I'll go out on a limb and say that the optimum altitude for an electric aircraft will be "as low as possible" for the forseeable future. Think "Piper Cub." Better technology (materials, battery weight/amp ratio) may result in bigger and bigger "Piper Cubs" with larger and larger ranges and payloads. Except that the requirement to continue a takeoff with one engine failed will limit that also, as far as regulated commercial transport ("airliners") is concerned.
Net - somewhere around 2000 feet MSL or 1000 feet above terrain and obstructions, whichever is higher. I think having "air to push for reaction mass" will dominate or drive all other aerodynamic considerations - airfoils and such will be optimized for "enough speed" with "as little as possible" drag. Fine-tuned for function - transport, loitering, private, professional.