This makes it so that as long as that button is being held, moving the cursors scrolls instead.

set_ergo_scroll.py

#! /usr/bin/env python

import re
import subprocess
import textwrap

RE_LOGITECH_ID = rb"Logitech MX Ergo[\s\-\w]+id=(\d+).*pointer"


def set_ergo_scroll() -> None:
    """Set button scrolling behavior for MX Ergo."""
    print("Setting ergo scroll...")

    inputs = subprocess.run(["xinput", "list"], capture_output=True).stdout
    ergo_ids = re.findall(RE_LOGITECH_ID, inputs)
    for ergo_id in ergo_ids:

        commands = (
            [
                "xinput",
                "set-prop",
                ergo_id,
                "libinput Scroll Method Enabled",
                "0,",
                "0,",
                "1",
            ],
            ["xinput", "set-prop", ergo_id, "libinput Button Scrolling Button", "8"],
        )

        for cmd in commands:
            proc = subprocess.run(cmd, capture_output=True)
            if proc.returncode != 0:
                print(f"Error calling xinput.\n")
                print(f"stdout:")
                print(textwrap.indent(proc.stdout.decode(), "  "))
                print(f"stderr:")
                print(textwrap.indent(proc.stderr.decode(), "  "))
                exit()

    print("Ergo scroll set.")


if __name__ == "__main__":
    set_ergo_scroll()