restart Android phone when power button stopped working
It would appear that the only way to reboot the device in the case where the phone is not rooted and when the power button is unavailable is through adb.

As per StackOverflow, it would appear that Google provides an adb binary for OS X that doesn't depend on any other developer tools nor Java, so, the following set of steps would make it possible to reboot the phone:


Have Developer options menu enabled within the Android phone
Enable USB debugging within Developer options menu of Android
Connect Android device via a USB cable to an OS X machine
Execute the following commands within OS X in Terminal.app:

cd /tmp
curl https://dl.google.com/android/repository/platform-tools_r28.0.1-darwin.zip -o apt.zip
unzip apt.zip
./platform-tools/adb devices
./platform-tools/adb reboot

Before executing the final adb reboot command above make sure to allow pairing between Android and OS X by allowing it at the Allow USB debugging? dialogue popup that would appear on the phone after running the adb devices command on the OS X machine.