This is excellent news then - we have at least one root cause to the panic, ie SPI access
To close this case, it looks like I may need to update from SDFat 0.0.7 to 1.0.16 as per @ScruffR’s comments: DeviceOS 1.5.0-rc.1 - SoftAP using SDCARD - problem (assumption being that 0.0.7 may not be using SPI TRANSACTION or has SPI.begin() in the constructor???).
I note that I had tried quite some time ago to use the updated lib but came to grief for some reason.
As I had already modified Adafruit’s SSD1306 lib for SPI TRANSACTION usage, this should be sweet as it is.
Will report back with results…