Combine protocols on a U/FTP Cat 6a cable (USB 2.0/I2C)
In theory, this should work. Cat6 cable is generally fairly high quality, low capacitance, low resistance, and high bandwidth. The twisted pair and individual shielding also helps. Host powered USB Extenders can use cat5 for up to 100m.

As for the capacitance for the i2c bus, you can get that information from the datasheet for the cable. Typical Cat6A capacitance is 330 pF per 100m. Look at this datasheet for an example.

To be honest, 6 meters is short. You shouldnt have any issue.