RS485 entering and leaving a PCB
Yes, you do want to keep the trace impedance on the board as close to 120&Omega; as you can, mainly because this affects how the signal propagates on the cables attached to it. Presumably these devices are being daisy-chained together, and the reflections caused by impedance discontinuities will accumulate to reduce your overall signal integrity.

There are online calculators for controlled impedance traces that include your configuration with no ground plane, for example the one from Saturn PCB is particularly flexible.

You have some other design issues to consider, however. RS-485 receivers typically have a limited common-mode voltage range that they can handle, so you typically DO need a ground reference in your cable.