I would create a zone for the power rails, and enclose the required pads within it.
The question about trace thickness is complicated, because it is about thermal dissipation. There are many factors involved, and can't be taken in isolation. For example, if the PCB is enclosed in a small box with no airflow vs a card in a rack with forced airflow. Effectively, the copper area of a trace acts as a heatsink, with more area providing better heat dissipation. Copper conducts heat well, but is impeded by narrow strips. FR4 conducts relatively poorly.
For the motor connections A1,A2 etc I would use the thickest trace that is allowed by the clearance requirement. I think with 2oz copper, the traces you have would be ok.
If you have a well ventilated case, I would suggest a fan for DRV8825 anyway, then the finer thermal considerations become a bit moot.