Loading Nexus Stock Marshmallow onto 3rd-Rate LG KitKat Phone?

So I have an old LG Spectrum II with Android 4.1.2 on it. It's about three years old at this point, but I wanted to do a little experiment. What if I loaded Marshmallow onto it?

My first order of business was, of course, unlocking the bootloader. Turns out, LG at the time was very against key-combos to reach the bootloader. Okay, so I can get to it from the computer with adb. No matter what I do, when I put in

adb devices

The phone won't show up. Performing

adb reboot bootloader

won't do it either. Does anyone have any suggestions as to how I can continue from here?