Ican think of two probable causes for the nose down problem:

-The wing generate a strong nose down moment at full speed.
-The propellor and motor provides thrust at the end of a long moment arm.

If the problem is due to the thrust moment arm, it should have improved when you modified the hull/fuselage with a higher wing position.

To check if it is the wing isgenerating a nose down moment, try reflexing the ailerons. (Adjust the linkage to raise both alierons, maybe 3mm up.)

Second try adjusting the motor thrust angle. There are two possibilities, both might work.

Adjust the motor thrust angle to make the slipstream blow down against the stabilizer. With full power the stabilizer should then generate a stronger down force at the tail, holding the nose up. I would try this first.

Adjust the motor thurst angle to push the nose down, in other words oposite to the above.

Enlarge the stabiliser with larger area, not a cure but the pitch down might become less severe.
Change to a T-tail layout, this put the stabilizer up in the slipstream, making it more effictive.