Ah got there in the end.
A working I2C scanner on the STM32WL, showing all my lovely chips reporting back, and then pulling the data off a pair of Adafruit Soil moisture probes, before pinging the readings up through my LoRa gateway, and through to my local Chipstack Server.
"Nice. Working, compiling project. Hm, i'll just add in I2C via CubeMX..."
>> make: *** No rule to make target '/Core/Src/dma.c', needed by 'Application/User/Core/dma.o'. Stop.