If you have to lower the bike, it is important to keep it as level as possible front to rear.
A 390mm spring is the same as a 350mm + 40mm spacer, and you can adjust the length of the spacer for a height change without it changing your spring rate.
It is your weight (fully geared up and with everything you normally carry) that affects the spring rate you need (sag). Although large changes in spring rate affect suspension action (obviously) it is fork damping (adjustments and oil weight) that will change the dive characteristics you don't like.
Have fun getting her dialed
Spring rate first to get the right sag, then damping adjustments.