It's bit more complicated than that. The balloon lift will decrease with pressure, which will decrease with altitude but so will aerodynamic drag. Ion engines design for satellite operation also prioritize low reaction mass usage and I am not sure the low thrust is a hard limit or a design choice. The device could also use vertical thrust to compensate for the loss of lift and then gradually rotate until it reaches orbital speeds.

It's probably impossible with current propulsion technology, but it would be useful to have a number when it starts to be possible.