For PCB isolation milling the thing that usually bites first isn’t X/Y stiffness, it’s Z. Copper clad is never perfectly flat and rarely clamped dead level, and with a V-bit a few hundredths of a mm of height error changes the trace width a lot. Whatever drive you pick, plan for probing (a clip on the bit, the copper as the contact) and a height map / auto-leveling step in your sender; Candle and bCNC both do it. That will matter more for your results than belts vs. screws.

On single vs. dual Y screws: if you go dual later, drive both screws from one motor with a closed belt loop rather than two motors. That keeps them in sync mechanically and avoids the racking/binding problem mentioned above. For a small PCB/laser machine though, one screw under a moving bed is simpler and plenty rigid.