The ant-6f contains a single coaxial port used to carry both the transmitted wave and the received wave, perfect for time domain duplexing. The TR6000 includes different ports for received and transmitted signals. Additionally, the power output of the TR6000 is in the range of -10 to -3dBm (very small). While it might be possible to design a custom RF switch board with low-noise amplifiers and a power amplifier for the link budget to close, it would not be ideal to test out a new PCB on a mission like this. However, GomSpace has a radio for UHF with one coaxial port that includes the needed amplification and switch for time domain switching!
This block diagram from the NanoComAX100 shows that it is capable of half duplex compared to the full duplex compared to the full duplex capability of the SKR Mark 3. The output of this radio is much higher at around 30dBm. The SDR Mark 3 is still being used for S-Band as the antenna includes amplification and a duplex filter.
Having both UHF and S being full duplex allows for two methods of communicating with the satellite. If S fails, you still have UHF. If UHF fails, you still have S. Given that UHF antennas are deployable, it makes sense to have S in case the deployment fails. Granted, many missions have only one comms link and work fine.
While it is possible to find TX only S-band radios, GomSpace does not have any that have a high enough data rate. Staying in the GomSpace ecosystem allows for better and easier integration of parts. I see no need to downgrade S to simplex.