Yes, heat treat would be better after fab. The problem with that is if you want to do more than one bumper at a time per batch, you have to build a fixture to bolt the bumper to in order to prevent warping from the relieved and induced stresses. The cost of those fixtures has to be amortized into the total run of bumpers and you have to analyze the cost versus benefit to the end user of doing so.
It is easier to just understand what it going to happen, design around that, keep the costs as low as is feasible for the most benefit to the end user and go have fun.
It is possible to solve every problem by throwing lots of money, tech, and skill at it. Few of those are a good return on the investment of same with regard to crap that is going to get drug through the rocks.