Have you considered to jettison some of the batteries during and/or after take-off?

Probably not just drop them, maybe a controlled glider or similar, plus what are the posibilities for an in the air charge by tether to another plane?

Final super radical idea, have some iflatable high chord wings for take off at low speed high drag and lift, then ditch them somehow once at altitude and can do a anouever with stored height to get speed. (Or a lift blimp).

As you say, the take off is where the problem is.