Nat -
Re your single engine taxy - why?
Option 1 - taxy out a long way (such as FRA, AMS) with both engines running in hot weather (such that the FADEC winds up the engines for bleed demand) a light load and arrive at the holding point with brakes too hot to take off.
Or
Taxy out single engine, arrive at holding point with 100kg extra fuel and cool brakes able to depart asap.
No brainer.