Integration or AppDaemon for wired RS485?

I have a 64 device with an AtMega CPU and now I am communicating with a special Windows program. I would like to use HomeAssistant for visualization, control and parameter setting. Some devices measure, for example, only temperature and control the relay according to the SET value and the measured value. I can communicate with these devices as MASTER about 2x per minute. But then I have devices that have controls on them and the operator can control them directly with the buttons, but it should also be possible to control them via HomeAssistant. It is necessary to communicate with these devices as often as possible, for example once every 2 seconds. The communication protocol cannot be changed to MODBUS. A wired RS485 connection must be used. My question is: how to make custom communication between HomeAssistant and RS485? Create a new Integration or AppDaemon? How do I run Integration or AppDaemon in the background to constantly monitor RS485 traffic and requests from HomeAssistant? Is it a good idea to use the existing Serial integration and modify it for my use? How often is code executed in Integration or AppDaemon? Please any information or suggestion on how to achieve the goal.
Thank you