It sounds like you have already done a thorough job checking the frame alignment and dropouts, which rules out some common causes of this issue. Here are a few other things you can check:

• Check the dish of the wheel: Make sure that the wheel is properly dished, meaning that the rim is centered between the hub locknuts. If the wheel is not properly dished, it can cause it to sit off-center in the frame.
• Check the axle spacers: Make sure that the spacers on the axle are symmetrical and that the wheel is centered between them. If one spacer is thicker than the other, it can cause the wheel to sit off-center.
• Check the derailleur hanger alignment: If the derailleur hanger is bent or misaligned, it can cause the derailleur to pull the wheel off-center.
• Check the frame alignment in relation to the seat tube: Even though the rear triangle may be aligned, the seat tube can still be off-center. This can cause the wheel to sit off-center in the frame.
• Check the dropout alignment again: It's possible that the dropouts were not perfectly aligned when you checked them the first time. You may want to double-check them using a different method or tool.

If none of these checks reveal the cause of the problem, moving the rim slightly to the right may be your best option. Just make sure that the wheel is still properly dished and that the tire has enough clearance.