A previous DYNAMIXEL Community member created this post documenting their custom solution for driving a DYNAMIXEL directly from RPi UART. Hopefully, their design will provide you some inspiration.
To continue YKW_R’s point, DYNAMIXELs use a 5v logic level compared to the RPi’s 3.3v, so I would recommend using a level shifter between the two, to prevent any damage that could occur to the RPi.