Thanks for more information about your plane.
As you states, with the nose down problem on aplication of power, it is probably a thrust line problem. Quite common to seaplanes with a high motor and thrustline.

First, T-tail and longer tail moment sound like a good idea.

The devil is in the details.
And sorry, my explanation was not that good.

Thrust angle, I would first try adjusting this to get the propellor slipstream down against the stabilisator. Thus increasing downforce at the tail. This worked on a design I did.
This is actually is angling the motor to push the nose down, but with the motor more or less straight above the centre of gravity/centre of drag the change in thrust angle have little leverage. The aerodynamic effect on the tail have a longer moment arm and more leverage.

Doingitthe other way around, the thrust push the nose down, but have to fight a decreasing tail down force as the slippstream change the stabiliser angle of attack.

It's a matterof balancing two opposite effects. The question is which is the strongest in your design. Tail downforce due to the slippstream changing the stabiliser angle of attack or the thrust force direct pitch moment.