Is there something special about yolov8 over later models (9-12)? It seems most of the research and working examples default to v8 despite it being 3 years old. Or just because it is what fits on this hardware?

Newer versions aren't open source, or at least have murky licencing.

Mainly because YOLOv8 is well-supported by the Rockchip/RKNN toolchain.

The goal here was an end-to-end RK3588S pipeline rather than comparing detector families: training/export, ONNX graph fixing, INT8 RKNN conversion, C++ postprocessing, and runtime inference across the 3 NPU cores. YOLOv8 has known-good export paths and Rockchip examples, so it was the most practical baseline.

Newer YOLO versions may be possible, but usually require more work around RKNN export compatibility.