There are many ways that lead to Rome, but all i can suggest is how i would do it.
I would make a universal system, so full DMX. If the distance becomes an issue, you can add a forwarder / amplifier later.
The connections to you cable should be like this :
And then you can use for instance :
To implement DMX on both ends. (Keep in mind that DMX has inverted logic compared to serial-protocol, so you may need to swap pins A & B if you connect to an existing DMX infrastructure.)
This library also supports RDM, which is a 2 way protocol that got added to DMX that allows for the setting up of devices that support it, using the same infrastructure, if you plan to use that, you may want to connect pins 2 & 3 to a digital output pin on your Arduino, so you can switch between modes of your transceiver, but for now i would use the schematic (i never use RDM)
You can test (and should test) the hardware part and the software part individually. Just connect the schematic to a serial port at each end, and send data across to verify it works. Then incorporate it into a sketch that sends and a sketch that receives DMX (the library comes with examples, use those ! )
That is what i would do.
Other options include sending your data using any serial format, and receiving that at the other end, for which you may want to have a look at Robin2’s
The advantage of that would be that you can simply lower the BAUD-rate to transmit over longer distance (half the speed is double the distance)
Drop the RS485 units that you have (or at least 1 of them), to modify 1 so it will be in receive mode is really quite hard, and a MAX485 chip is really cheap and easily available.