Breadboards are wonderful tools that let you very easily spend 2 hours chasing your tail by just swapping 2 wires... Anyway, progress, after the stm32 (f303k8), I have now the AR488 code https://github.com/Twilight-Logic/AR488 running on an esp32 (via SN75160/161 transceivers). Now let's see how to make this GPIB "dongle" wireless...
OK making Bluetooth SPP work on Linux ils a bit tricky, but it werk! Note the comm port is rfcomm0 in there. I guess I need some udev black magic to make it "just work™" but ok