I’m not sure there is a way to draw a dotted line in schematic (although someone else may know a trick), you might look at the schematic frame (a drawing title box) which I think can have a box around it. That may work to isolate the circuitry on schematic. It can be found in the core parts bin Schematic view (5th from the left). That will create a title block (where you could indicate this is optional circuitry) and create a solid (I don’t think dashed is an option) line around the optional circuitry. As you say the parts will still show up in breadboard and pcb, but they would need to if you wanted to install the parts.
Peter