Reception of symbol lines from a sequential port

This is not a course or a diploma! I haven't been a student for a long time. This is an attempt to write another free software for "home" car diagnostics. And I'm not very good at Python. That's why I'm asking for help.

Source data:

There is some device that is connected to the computer via a USB port. The computer and the device exchange data in symbolic form. In fact, this is a classic connection: the device plays the role of a server, and the computer plays the role of a terminal. If this tells anyone anything, then this device is an OBD-II adapter (On-Board Diagnostics - a device for diagnosing cars) ELM327.

The system works according to a very simple principle - the server returns a response to the client's (terminal's) request (command). Here is an