Are you sure that the Synapse will not take the guards you'd want? Ask the supplier (Evans I assume) to fit them so any fettling will be handled by them. Not sure about the rack mind you. As above, I have 28s with Cruds on mine and there's loads of space so I'm not sure why you think 25 is the maximum.
The CAADX seems like the choice but check the geometry is OK as it will be different to the Synapse.
The CAADX seems like the choice but check the geometry is OK as it will be different to the Synapse.