I think that is likely the winning path. When I first looked at this that was my approach, basically a flat sheet of copper with the inverter laid out on top of it. Basically a 10 x 16" copper heatsink with holes in it where vertical components sat, capton tape on the back flex circuit and D9 packaged semiconductors on the other. My battlebots motor controller (which really was just a 24DC to 24V PWM converter at 200amps[1] which was in the ball park (power wise) but FETs don't really like operating at really high voltages so I was looking at an IGBT version.
As I was more concerned with power dissipation early on, more surface area is great for convection.
