Even on my mid-drive Haibike, the trick to going up hill, with an eye towards battery power economy, is to lightly spin the pedals.
And this is done by gearing down to the appropriate front chain ring to the smaller sprockets as well as shifting to the biggest rear gear you have. Think light pressure on the pedals. If you have a motor output meter, like I do with my Yamaha e-drive, you can actually watch how much power is being put out by the motor as the power meter pegs out. When I am going upgrade or uphill, I know I am maximizing the power output to be it's most efficient when those power meter bars are at 4 bars or less. Pegged out; well, this is where maximum motor watts from the battery to the motor are giving way to heat in the motor, shutting it down when it overheats.
I'd not worry about losing the pannier bags; it's good to carry the required gear you may need to repair a flat or carry home some groceries, etc.
Instead, focus on downshifting to a small front crank sprocket and large rear cassette sprocket and spin lightly as you can; within what your particular drive gear ratios will allow you.
Also, for maximum mileage and overall efficiency, take a look at the maximum air pressure allowed for those tires and air up to within 5 psi of that max allowance. (this pressure rating will be stamped on the tire's sidewall) Low tire pressures result in more watt/hrs output required to roll those tires.
Good luck and let us know how it works out for you. I think Pmcdonald is onto identifying your issue here.