Well ChatGPT solved my problem....increasing the spreading factor to 11 has suddenly made everything work. Not sure why such a high SF would be needed between two devices on the same desktop? Will soon find out.
In the meantime, the following table gives some interesting perspective on the transmission time for a 19byte payload.
SF Symbol Duration Payload Symbols Total Symbols Airtime
7 1.024 ms 37 51.25 0.052 s
8 2.048 ms 37 51.25 0.105 s
9 4.096 ms 37 51.25 0.210 s
10 8.192 ms 37 51.25 0.420 s
11 16.384 ms 37 51.25 0.839 s
12 32.768 ms 37 51.25 1.678 s