Okay, so now I feel like an idiot. Looks like I won't need to cut down the steerer or mess with the star nut at all.

I received the frameset last week and up until just now I never actually put a stem on there to see how much steerer would be left. When I received the frame the steerer had apparently already been cut, but it came with a tall top cap, a 20mm spacer, and a few 5mm spacers all stacked up with the compression bolt on top to keep the fork from falling out the bottom. I assumed the previous owner had it set up for a taller stack height due to the tall top cap, but I was wrong.

Turns out the previous owner had it cut for just the ~15mm top cap and a stem. The stem is about 1mm above the top of the steerer when that 15mm top cap is installed. With the new headset I ordered and the 9mm top cap I will have enough room for just the top cap, stem, a 5mm spacer on top, and then the compression bolt. It's perfect right where it is and I don't have to do any cutting.

Sorry for wasting your time guys. I still appreciate you guys trying to help.