Your attic is correctly insulated.

What you have is a typical ‘cape cod’ type room where there is insulation on the floor behind the knee walls, insulation up the knee walls, then insulation in the rafter from the knee walls to the ridge.

The only thing that is missing is proper ventilation and this should be done passively without the use of a powered vent.

You need to install vents in your eaves and vents in the attic area where the rafter meet or in the roof ridge.

You also need continuous rafter vents from the top of the knee wall to the ridge to prevent condensation.

But that’s pretty much it.

Slight mildew problems don’t even need to be treated. Once they are robbed of moisture, the mildew will cease to grow.