It would be helpful to those of us responding in this thread what step-thru bike you have. Is it the City, Rover or Mini? The City has a direct drive motor, the Rover and Mini have a geared hub motor. The geared hub motor has twice the rated torque as the DD motor. I’m not too sure the Bolton kit would do you much good on the DD motor. I would also worry a bit about burning out the motor if you climb this steep hill often.
Another thing to keep in mind is as the battery voltage drops, so does your performance. Perhaps below 75%-50% charge the battery won’t be able to sustain your climb and you’ll be back in the same boat you are in now. Sometimes getting off the bike and walking it is the best option. There is a walk-mode function to aid this effort.
If you have the Rover or Mini, the Bolton kit may be a good option for you. If you have the City, that may not be worth the dollars and effort spent. I believe the Bolton kit cables plug into the Rad Bike cables, you just need to remove the original controller box and display and mount the Bolton equivalent.
Good luck, whatever you decide!
RangerDave