Hi, I understand the project's requirements. I have previous experience in a similar project where I created a desktop app for Windows with a custom installer where you can have an executable file with your own logo, app name and company information, app sends a string over ethernet to the microcontroller to turn on or off the relay, the status on the app is only updated after receiving the response from the microcontroller which means that your UI will never be out of synch.
On the hardware side, I can provide you BOM, PCB design for plug and play solution.
I'm available for any project related discussion.
Looking forward to working with you.