I have had a similar problem when I first got my radwagon. I have extensively modified it and have also installed a Bolton kit on another bike, so perhaps I can add a few things to check and possible options.
First of all, it seems odd that you can't get up the 10% hill with the direct drive motor. One of the things I would do is to try pedaling without assist on flat ground. Are you feeling excessive resistance? Is the drivetrain/brakes rubbing in a weird way? You expect to feel some resistance with the direct drive motor from the rad city, but it should be possible to pedal in all gears on flat ground. A mechanical problem could be really affecting the bike and might be hard to detect unless you turn the assist off. If in doubt, have a local shop check it. For context, my 2016 radwagon (which uses the same motor) was just about able to get me up a 20% incline in the hill where I live. I absolutely could not get up with cargo though, so I had so do other things to my bike (yes, I am aware that the incline is beyond the max recommended by Rad).
Second, what the Bolton kit does is essentially upgrade the controller for your motor. The display changes as well and you get more configuration options, but that is secondary. The main thing is that the upgraded controller will let more amperage to go through the motor, this in turn means more torque, which is exactly what you need to get up the hill. I bought a Bolton kit to install on my husband's Radrover because it was also not doing great while going up my monster hill when loaded. The kit is pretty straightforward to install, but you do have to be willing to tinker a bit with regards to attaching things on the bike, etc. The new controller made a huge difference regarding the torque of the motor. Now, the kit is designed for the radrover (a geared drive) and you have a rad city (direct drive) but it will almost certainly work fine. It is also extremely unlikely the new controller will damage the direct drive motor. One of the main reasons that people use direct drive motors is their extreme durability and reliability. In particular, they are good at shedding heat, which is the result of sending more amps through the motor.
The other advice is to get going as fast as you can (full throttle!) as you approach the hill, that is not just because you will get farther from the inertia, but also because the motor is more efficient at the higher speeds and will transfer more power to the wheels, so you get an advantage in both senses. This means (counterintuitively) that you might want to be in the gears that allow you to go faster but make pedaling harder as you approach the hill and gun the throttle.
Good luck!