Lower NVIC priority number means higher priority.
For UART, FIFO Mode must be enabled.
When starting a new FreeRTOS project, you must manually increase the total heap size. Hard faults may mean a thread is running out of stack memory.
With UART DMA, calling HAL_UART_Receive_DMA() takes a long time (maybe, 1ms). Do not call it inside an Interrupt Service Routine (ISR). Call it only once to start the DMA receive.