This reminds me of that old discussion from three years ago.

Here is my attempt. The sample code with embedded Echo is this:

Module[{}, x = Sqrt[2]; If[myEchoQ, Echo[x]]; N[x]]

The flag is called myEchoQ. When myEchoQ is set to False, no echo is generated and no unnecessary evaluation of x is made, because If has the HoldRest attribute:

myEchoQ = False;
Module[{}, x = Sqrt[2]; If[myEchoQ, Echo[x]]; N[x]]

When myEchoQ is set to True, an echo is generated:

myEchoQ = True;
Module[{}, x = Sqrt[2]; If[myEchoQ, Echo[x]]; N[x]]