How to go about reducing sugar in a butter cake recipe?
According to Shirley Corriher, as reported in Fine Cooking, the sugar should weigh slightly more than the flour:

The sugar should weigh the same as, or slightly more than, the flour.
Remember that this is weight, not volume. A cup of sugar weighs about
7 ounces, and a cup of all-purpose flour weighs about 4-1/2 ounces.
So, if we're building a recipe with 1 cup sugar, we'll need about
1-1/2 cups flour (about 6-3/4 ounces).

Sugar plays a very important role in creating the structure and bite of cakes, and also helps to retain moisture and inhibit spoiling, so you cannot simply reduce the quantity without limit to control the sweetness.
That being said, most recipes have a certain amount of tolerance. You can probably reduce the sugar by about 20 to 25% without completely altering the nature of the recipe, although the crumb may suffer a little.

Vanilla and rose are delicate flavors. If you are making a yellow cake with butter, and egg yolks, they may be competing and masking the flavor.
Using a white cake base may allow these flavors to show through more clearly.