And now an entry in the "Plan to throw one away" section.

RF Multiplexer, 8 inputs, 1 output, BNC-connectors, TE HF3 relays specified to 3 GHz, an ULN2803A to pull the relay-coil, and an SPI I/O expander to drive the ULN - should be easy - right?

Well no, PCB trace-geometry does strange things beyond VHF. I clearly don't grok UHF very well.

Onward towards version 2! (any thoughts and advice on simulation or trace-geometry optimizers appreciated!)