USB full speed routing requirement
In accord with many USB layout guides, it is still recommended to route FS traces as 90-Ohm differential transmission line. However, it is nearly impossible to make 90-Ohm trace on a cheap 2-layer 1.6 mm thick PCB. It will take a lot of PCB space if done correctly.

FS signaling is not something of very high frequency. The carrier is 6 MHz, edges, let say, 24 MHz, so the wavelength is ~600 cm. So 1/10 of the wavelength is a lot, 60 cm, and 3-6 cm should not make big impact on signal waveform or essential impedance mismatch. So route your traces as a pair of ~0.5 mm traces 0.2 mm apart, and forget it.