I'm not terribly familiar with Linux and not at all with the A64+. But I'm pretty sure the power button won't help you if `poweroff` doesn't. Can you`shutdown now` ? If that works, I expect it would cause an orderly shutdown and halt the system without rebooting. At that point you could disconnect the power.
Do you have an option to use journaling? I don't know if that's ideal in your situation but it might be safer if you can't achieve an orderly shutdown.
Do you have an option to use journaling? I don't know if that's ideal in your situation but it might be safer if you can't achieve an orderly shutdown.