Struggling with serial communication over RS 485 using modbus protocol
I don't have great knowledge about the Arduino platform, but I have worked with Modbus over 485 lines. All processors I used, requires pull-up resistors on the Tx-Rx lines to the MAX485. Maybe Arduino board implements internal pull-up resistors, can you verify this?

I have much doubt about the manufacturer's specification on the "start byte" and "end byte", this does not appear in the specification of Modbus protocol.

And finally, 60-70 nodes is a large node number... maybe you must check the specification for Modbus over serial line.

You should also be careful about the way to end the transmission line and how to perform wiring. Check the specifications.

Here is an example of a circuit that I used for Modbus over 485.



In this case, is an isolated line, but is the same principle.