Your structure is clean, but the import issue is normal, when you run a module directly from inside the package (like lanmessage/starter.py), Python no longer sees lanmessage as a package, so absolute imports fail. The usual fix is to avoid running package modules directly and instead create a top-level entry point (like your start.py) or use python -m lanmessage.starter so the package namespace is preserved. If you want starter.py to be the main entry, running it with -m is the most canonical and keeps everything inside the package working correctly.

It looks like ser.write() needs a bytes object, not a list. Try encoding each value to bytes before sending, or join them into a single string and encode it. For numbers above 255, you can use struct.pack() to send them as raw bytes.