I'd get a 4" backing plate/pad setup for those deeper scratches. No point wasting time when doing it commercially and that's the most efficient (and most effective too) way to do correction with the PC.
Noting that everyone has their favorites and there are few *bad* products out there these days, I'd look into Hi-Temp's Heavy Cut for your aggressive product
AVAILABLE HERE . We used lots of Hi-Temp stuff (via Cyclo, not too different from a PC w/4") when I had our dealership. This oughta finish down nicely enough that the #66 will be an OK follow up.