All good points Warren. I did some checks of isolation using a typical DIP relay, type HK19F. DPDT
I made a small jig to hold the relays and measure isolation across contacts etc.
Typically it was -34dB across open contacts,
-32dB from one set of contacts to the other. (common terminal)
-41dB from open terminals on each side
-39dB from common of one pole to open on the other pole.
-48db from common to coil.
At 30MHz. Better as freq drops of course.
That's without effects of a PCB and various other traces.