Power is transmitted through the prop (obviously) & the power load should the same for that 3B prop, or an appropriate 2B -- except that the 2B will have to be longer, & thus have more mechanical advantage, making any torque effects greater. However, most tail swinging problems come from the spiral slipstream off the prop, & not so much from torque effects (or the very misunderstood P-factor), so a prop change won't be very noticeable (other than improved acceleration).

Getting rid of the flywheel will help somewhat by reducing gyroscopic precession. However, you must compensate for the C-of-G shift by adding non-rotating nose weight, or (better) moving the engine forward, if possible.

Most tail dragger problems are reduced by the things that you have already tried (toe-in etc), & most go away with practice. For example, are you holding full up elevator at the beginning of the take-off roll, to keep the tailwheel planted until the rudder has some bite? Steering corrections should be anticipated and pressure applied to the rudder ahead of time -- note pressure, rather than gross rudder movements.

Tail draggers are a bit more awkward for beginners, & short-coupled tail draggers are worse than long-tailed types. The H9 Mustang is still a Mustang, despite the aero do-dads that numb its flight characteristics, so it behaves like a Mustang on the ground.

Just something to live with.