Electronic – Should I consider using RS-485 for the next project

I wouldn't trust a standard zener to protect a line running outside. Can you show the circuit? Are you sure it doesn't mean a TVS diode anyway?

You could use a single (say 6-10V) TVS on the lines as you mention.
A more expensive but more robust solution would be to use a three stage protector like this (or make your own)
It combines a gas discharge tube with a series impedance and a TVS. You get the benefits of the gas discharge (high current capability, high breakdown voltage, slow) with the TVS (fast, low clamping voltage)

What's the data rate of your RS-485? Since your cable isn't twisted pair you may have to limit the speed to 100kbps and use short cables.

But IMO the mismatched impedance isn't the real problem.

"The characteristic impedance of an RS-485 cable is 120 ohm according to the standard, but cables designed for high frequency operation usually has an impedance between 135 and 165 ohm, which limits the capacitance to approximately 30 pF/m." (source).

You don't mention voltage or current for the motor's power, but at 200W both may be too high to avoid trouble. A high current will inductively couple with the data wires, a high voltage will do the same capacitively. I expect you may see serious disturbances.

The cable just doesn't seem to be fit for the application. It should have been a double twisted pair at least, preferably with different twist rates.

edit
All will depend on how the motors behave and what your data look like. Have you had problems yet with the deployed cables? Eight noisy amperes coupled over a long stretch doesn't sound good. Bad looking waveforms in itself are not necessarily a problem if the receiver can properly detect edges. What are you sending, UART format? If you have continuously glitches sooner or later they might be seen as a start bit, and you'll have a ghost byte in between your actual data. Not only the data format is important, also what happens at the higher protocol levels. Is there a checksum or CRC? Acknowledge? Can faulty received data be retransmitted? If you would encounter communication problems I think this is the level where you can attack them.
It's also possible that for the installed cables no problems are reported at all. The 57kbps may help here. And maybe there is an error detection/correction mechanism in the protocol.