Magnetic induction on data lines from DC power lines
I have no particular experience with such simulation, but empirically it makes sense to add a ferrite bead to limit the inrush current spike and/or a cap to absorb a voltage spike when the current stops:







simulate this circuit &ndash; Schematic created using CircuitLab

In your case specifically the cap should be useful, since your cables already provide enough inductance if they're long enough. Adding the cap will shunt most of the spike which occurs when the current stops, instead of letting that spike go into your RS-485 line via parasitic capacitance. Or, you can decrease that capacitance by increasing the gap as you have already seen.