I am sorry, but I have no experience on working with coreml

but according to the source code:

https://github.com/ultralytics/ultralytics/blob/main/ultralytics/engine/exporter.py#L1121

I think the CoreML export from ultralytic yolov8 should already include the NMS module (I believe is convert to a CoreML supported setting/module)

And so you can try simply do export with nms=True (which I read form their doc, the nms switch is ONLY supported by CoreML)

https://docs.ultralytics.com/modes/export/#export-formats

Hope this help