Turns out the STEP file had the exact same positioning as the STL I’d already imported as a reference, so it was a drop-in replacement in my design!
Here’s a screenshot of the current progress, now with a thicker body for rigidity, the motherboard mounts from the standalone case, and the hinge mounts!
I guess the next step is measuring the positioning of the holes in the midplate kit (possibly also using the measurements from this accessibility guide if they’re accurate enough), since those aren’t present in this model yet. Then it should be good to take a first pass attempt at printing!