The eye diagrams in your attached project are not really open, so I made changes to the first channel to get better results. The symbol rate was too high for the optical receiver. Even though overall you are correct about the symbol rate. The symbol rate over each channel is equal to the bit rate.
The other aspect of your project that I saw was that the signals have incredibly high powers >100 W! Typically, in a optical system the powers are around 5-10 mW. The nonlinearities in the fiber are causing the distortion of the signal, which DCF cannot compensate for. I’m also pretty sure the fiber’s would get pretty hot at these powers. Can you lower the power of your frequency comb generator? In Transmitters Library/Optical Transmitters there is a WDM Transmitter component that could possible replace your frequency comb generator.
Regards