A Restore will install whatever the latest iOS version is available for your iPhone, but I am confused... if you do not want to "erase all content and settings" then why do you want to restore?
... the "erase and reset" feature is not what I need. I simply want to erase all data on my iPhone, while conserving its firmware
"Erase all content and settings" erases securely but leaves iOS firmware intact. It seems that doing so accomplishes this goal. Am I missing something?
A Restore will Install the Latest Version of iOS for your Device.