Apple is pretty strict with security measures. Even well-established tools like dtruss are hard to run due to SIP (system integrity protection); an intercepting kernel module similar to ones we use on Windows and Linux for Serial Monitor would be nearly impossible to get notarized by Apple, and so on.

So for the time being, capturing serial traffic on macOS can only be done at the physical layer using taps or USB-to-Serial adapters.

I know that the dtruss (dtrace) doesn't see to function on my M1 Mac (while it still works on my same-version M68K laptop). Is it just the Apple tax for security?

The Serial Monitor plugin runs on Windows/Linux -- so if you can run your app on Windows/Linux and capture the traffic there, that's an option.

Also, you can use a Serial Tap sniffer; no paid capabilities are required, and "Serial Tap" plugin is always enabled.

For a half-duplex RS485 link, you can also use a second USB-to-RS485 adapter and read from it using the Serial plugin.