For all high current PCBs, I use minimum 3+3oz, most of the time 4+4oz, Top and bottom.
Then leave the solder mask exposed for 80% of the Cu strip, makes a HUGE difference
with heat !! Also allows me to Solder a thick wire the full length for heavier currents.
I add two "stake" holes at each end in case I need to do it. This set up easily carries 20A
with very little Cu heating. PLUS, I always add a two isolated ~3" strips along an edge
for testing, one exposed, and one masked. It verifies the quality of the PCB.