Just an extra tidbit…
The larger arduinos, often have more than one hardware serial port (e.g. MEGA 1280/2560 have 4, 1284 has two).
These will allow you to use the Serial Monitor as intended, while you connect your custom serial interface on one of the ‘extra’ ports.
If you’re a real masochist, you could use a SoftwareSerial port on the smaller Arduinos, but can paint yourself into a corner really quickly!