My supplier has this stuff, but it can only deliver PCB's with 1Oz copper. To handle this much current I need thicker copper.
Er, why? The current-carrying capacity of a track is dependent of its cross-sectional area (you can make the track wider) and its allowable temperature rise. Its temperature rise is dependent on how well it can dissipate heat, and on Aluminium, a track can get rid of heat much better than it can on FR4. Aluminium power plates routinely handle well in excess of 50A with standard 1oz copper.
Do you think the ACS758 running at 150Khz sampling is unsufficient?
Oh no, that's perfectly sufficient!
Basically I am using PWM (Arduino about 600 Hz)
Then you're in the grey-area, and I'd use active switching at 12V with a series gate resistor (begin with 10 ohms) to limit dV/dt. If you have any wound components in your load, it could be quite noisy at 600Hz. With active gate drive, if you have sufficient PWM resolution, kicking the frequency up to 20kHz would cure that. But if sound noise is not an issue, it may be more efficient at 600Hz.