Just pull the parking brake and accelerate until you feel the car slightly raising and then drop the parking brake.
Eventually you get a feeling for it and drop the parking brake before it’s “fighting” the accelerator.
This might sound trivial to some, but I know several people that never use the parking brake in these situations and instead do a manic race with their feet and the car drops a couple meters back and they over accelerate to compensate.
Do you have a computer telling you that speed or are you just making a guess? Because I find it unrealistic to be +30 kph on flat ground with a mountain bike for 30 minutes.
I say this because I have a gravel bike and can only keep +30kph for long periods if I’m on a slight incline and I’m pedaling with a purpose (not full sprint, but you wouldn’t see a commuter pedal that hard)
On average people in commuting bikes will most likely be at around 15kph, low 20s on descents.