But what if you get it back? Or if you just keep it?

There is a chance that you have Pegasus on there, and I wouldnt want a phone without the detection of this.

You attempt to flash your full backup to it. And maybe then read it back if you can for verification that it was actually written to memory, but that probably won’t be possible when using fastboot. That’s all you can do that’s reliable, to some extent.