Surely it isn't impossible to devise one-way data flows that provably work for remote sensing? And yeah then you have to send somebody to fix stuff if something is off..

Like something that would work but not not scale would be one computer writing data to an updating qr code and another reading it. Surely something like that can be made (and probably already exists?) on the cable level?

Just go dumber

Set up a monitor with the data values you need to monitor

Point a camera at that monitor.

Camera feed is remote accessible. Control software is not.

Want alarming? There's systems designed specifically to send texts or make phone calls when signaled electronically.