Deal with the ethernet port issue by purchasing a 5 port ethernet switch. Maybe the rest of your issues go away?

I looked and Python has the library support for the GPIO and to do background threading to poll pins. My preference would be to go with a JVM language like Kotlin, but then I’m a programmer. Python, from the little that I’ve mucked about with it is really just one step in complexity from scripting. Maybe even easier, because some things in shell scripts are super difficult to do.

Maybe then you need to move one stop up from scripting into something closer to actually programming. I’d be surprised if Python doesn’t have the library support on a Pi for dealing with both serial and GPIO I/O.

the end stop in external to the serial communication

Does this mean that you have some kind of other signals or pin-outs? If so, this is starting to sound like a great project for a Raspberry Pi, because the GPIO pin array can handle that.

Keep in mind that it has been decades since I last used Kermit, but I’m pretty sure the use case it was originally designed for was…

Connect to a serial port, which had a modem attached. Talk to the modem and get it to dial a number. Presumably, the remote end answered and the port attached to its modem would issue a login prompt. Negotiate the login and then issue a bunch of commands to change directories and then launch Kermit on the remote system. After that Kermit to Kermit communications took over until you terminated the session. Finally, log off the remote system and hang up the modem.

All of this stuff could be done via scripts. I seem to remember that it would actually wait for a response, and then parse the response in the script. I don’t remember ever doing polling loops.

If you’re on a *nix box of some type, it’s totally possible to open up a serial port for manual I/O even in something like a bash script. Even if you have to reverse telnet to a terminal server.

Kermit on top of FTP can work really well. Kermit has its own communication and transfer protocol, IIRC, but updates in the 1990’s allowed it to be used with TCP/IP and FTP. So you can write a script to log into a remote system, run some commands and then initiate a file transfer. The scripting allows you to wait for responses and act on them.