Resolved! We need the register-based low level SPI R/W drive for STM32C031C6
1. CubeMX only allows HAL or LL for SPI and other peripherals. STM32C031C6 itself provides the good SPI clock rate. SPI LL API function only allows two-byte R/W.. The slave STPM34 Meter IC requires 4-Byte SPI R/W. So we have to choose HAL SPI API ...