Although I think the Arduino acting just as a conduit for the data is not really necessary but I guess you have some reason for the hardware choice. The Raspberry Pi is also able to talk ModBus RTU (libmodbus, pymodbus, etc.) so you may eliminate the Arduino if that's the only task for it.
What's the reason for using MQTT? From my point of view it just adds complexity. Is it because NodeRed has a MQTT module available? It has a serial module also which might be used. Is NodeRed already predefined?