Your serial connections between the two boards and settings in the Simple Motor Control Center look fine.
From your pictures, it looks like you have the Raspberry Pi 3 Model B version. There is a known UART speed issue with the Raspberry Pi 3 where uart1 is dependent on the CPU clock, meaning any changes to the core clock will affect the baud rate. I tested your code again, but this time using a Raspberry Pi 3 Model B, and implemented this workaround. I got your code to work with the SMC. Did you try implementing something like that to address the issue?