STM32 <-> Raspberry Pi3 SPI communication

Hello dear freelancers. I need firmware for STM32F103C8T6. The electrical circuit of the device is shown in the attached file. As you can see, the microcontroller is connected to the Raspberry via an SPI interface (note - the CS (SS) line is not used). Also, two UART-RS485 converters are connected to the microcontroller (in the figure, the connection lines are circled by ovals). Firmware Requirements: 1 - upon receipt of a data packet from Raspberry transmit this packet except for the first byte to one of the serial ports of microcontroller (UART0 or UART1). If the first byte of the data packet = 0, then broadcast in UART0, if the first byte of the packet = 1, then broadcast in UART1. 2 - when external data arrives at one of the ports (UART0 or UART1), transfer this packet to the SPI interface by adding the port number (0 or 1) at the beginning of the packet. Please pay attention to a few points. 1 - the task is very urgent. A preliminary result should be obtained within 4-6 hours. 2 - the project budget is limited and, unfortunately, is not subject to discussion. 3 - in the future there will be similar projects (1-2 per month) with a much larger budget and not so tight time frames. 4 - money will be released immediately after testing the firmware on my equipment (literally within one day). Thank you for your attention. I look forward to the suggestions.