STM32H743 FMC with ST7789 doesn't work without delay
Found the issue, it was a missing volatile on the pointers to the FMC memory or as in STM32HAL syntax __IO - the code should be:
static __IO uint8_t* fmc_command_pointer = (__IO uint8_t*) COMMAND_LOCATION;
static __IO uint8_t* fmc_data_pointer = (__IO uint8_t*) DATA_LOCATION;

Now why this works in my STM32CubeIDE is a mystery, probably a gcc compile flag