It seems the issue might be around the data buffer causing the stalling as explained in this thread Maximising cellular data streaming rates on Boron
Once I add a 400ms delay between the writing of each packet, then it works, it doesn’t stall after Package #11.
Interestingly if I try to send the second time without hard reset (eg reboot), the second time it goes a lot slower. I am not yet sure whether this is caused by the delay() function behaving differently the second time or something else.