Mounting or unmounting a filesystem won’t make a difference for drive longevity.
If you want to keep your backups secure, you want to keep them offline, so if you get ransomware it doesn’t encrypt your backup too. (Or if you just mistype a command and target the wrong device, folder, etc.)
But drive motor starts and stops are when the most failures occur. So the ultimate question isn’t how to make a drive last longer, it’s how you plan to handle it when the failure inevitably occurs.
That’s where it does the voice processing. The only processing it does on-device is the wake word and taking commands. Actually figuring out what you mean is done in The Cloud. Doing that on-device would not only make the devices significantly more expensive, but they would also rapidly become outdated.
The rest of your complaints are valid and I’ve experienced them all myself to boot.