The FT Simple Cub is a larger plane than the Tiny Trainer and they should not be compared for motor selection. There's a reason the Tiny Trainer is in the "Mighty Mini" section and the Simple Cub is in the "Swappable" section. ALL the Mighty Minis fly on smaller higher KV motors with 6" or smaller props. Everything in "Swappable" flies on larger, lower KV motors with 8 to 10" props.
If you don't want to buy the B or C (proper sized motors) from Flite Test, buy something in the 22xx/1000kv size range like this (which is between the Flite Test B and C size)...
https://www.amazon.com/gp/product/B0799HRLGZ/?tag=lstir-20
For brushless motors, the first 4 digit number can be broken into two parts **## where the ** denotes the stator width and the ## denotes the stator height (so the linked example as a 22mm wide by 16mm high stator). So larger numbers mean bigger motors which equates to more torque and more current draw.
The "KV" number roughly equates to the motor speed per "S" (cell count) of your battery. So the linked motor, at 1120kv, would rotate at about 3360 rpm (unloaded) on a 3s battery.
To swing a bigger prop, you need a bigger motor, and generally these also spin slower.
There's more to it than this, of course, and I am by no means an expert in it all. But I do know the linked motor should be sufficient to fly the Simple Cub on a 9" prop.
The motor you are using, with a 6" prop, is NOT sufficient.