Don't do it!!!
I bought a road bike recently and find the road position too uncomfortable for commuting in the city, especially for reaching the brakes fast and slaloming between cars. Therefore, I would like to convert it to a flat bar.
If your brake hoods are positioned correctly, you should be able to commute sitting up with your hands on the hoods, and brake from there. Braking is certainly not as strong from this position, but it's effective enough, and is quicker than getting into the drops to brake.
The problem with a flat bar for commuting, compared to a road bike with drop bars, is that the flat bar is wider. So when you say slaloming between cars, I think you'll be restricted by the extra width.
I have commuted with drop bars this way for 30 years in heavy traffic, and never had a problem with being able to brake hard enough. And that's using traditional cable actuated rim brakes.
If you have not been used to braking from the hoods then it will feel strange at first. Practice, and maybe build up some hand strength in the process. And ride with a little more caution until you gain confidence in your ability to stop. At best, you'll have developed a new skill and saved your money. At worst, you can switch to a flat bar later (with a different bike, because the geometry will be wrong on this one).
Finally, it could be that your brake hoods are positioned too low. They should be set up so that braking is possible from the drops and the hoods.