Using 12v to supply a Raspberry Pi with a switching regulator
I would not worry about the spikes on the supplies. Assuming your 12V supply is any good (properly regulated) the switching of some relais will not cause much spikes.
Also, that LM2596 board will suppress most of these spikes.

The 5V output of the LM2596 will have some supply ripple (since it's a switching regulator, which is a good choice in this situation !) but nothing the Raspberry Pi cannot handle.

I think it will just work without any problems !

One note: don't forget to adjust the LM2596 board to output 5V before connecting the RP.