After reading all this, I can only wonder....
WHAT is preventing you from using a larger prop?
It's an 8-FOOT airplane!
An 8 inch prop can't make it move.
A 16 to 20 inch prop, a large motor with a gear box, a basket full of batteries.. That's "approriate" for this task.
The configuration police won't be coming by to shoot you down if you go larger, in the prop, the motor, the battery.. anything at all..