Detachable "debug" portion of PCB?
Don't make the middle part too small. PCBs are produced in a 'frame' and removing the PCB from the frame has the danger that the debug portion breaks off easily in the last stage of manufacturing. I would use two sots and leave a small connections at the outside and a bigger one, with routing in the middle. Below is a picture I took from the WWW of PCBS in a frame.

I would also make some solder points that you can reconnect the debugger later on.