FL1000 is likely not possible using electricty. Electric power is converted into thrust via propellers pushing against a working fuild (air). The less air at fl1000 means less working fluid. A combustion engine adds to the air, generating thrust not just by propellers but through rocket forces: heating the working fluid and ejecting it at speed. To do that with electricity one would have to convert the electricity into heat, a massively wasteful process. The only way to get the necessary heat without burning something would be nuclear tech a la Skyfall or project Pluto.