I managed to get throttle and brakes correctly mapped to the triggers on my DualSense with the following adjustments to config.ini:
brakeAxis = 3
brakeAxisDeadzone = 0
brakeAxisNormalized = 1
brakeAxisInvert = 0
throttleAxis = 4
throttleAxisDeadzone = 0
throttleAxisNormalized = 1
throttleAxisInvert = 0
Essentially you just have to decrease throttleAxis and brakeAxis by 1. This might be because, by default, the right stick is also picked up on the default throttle/brake axes.