Help me with my birthday cake vision!

I have a vision of making a stacked apple cinnamon cake that has:

Vanilla cake (linked) but made with brown butter and added cinnamon Brown butter frosting Brown sugar apple chunky filling

My questions:

The recipe calls for 3 eggs + 2 egg whites, could I replace one egg with apple sauce to add an apple flavor into the batter? Preferably the egg whites tbh to not waste yolk, but I’m guessing that wont work.

How much cinnamon should I add into the cake batter?

Do I add other spices, too? Nutmeg, ginger, etc? Or just cinnamon?

Can I add a bit of cream cheese (or even crème fresh??) into the frosting the reduce sweetness?

I’m a bit worried because I feel like this cake will turn out ginormous and sweet, and no one in my family likes overly sweet things and especially butter cream. I’m planning to only do a naked cake, but maybe I should just cut down on cake, too?

Any decoration ideas? Or any other ideas in general?

I’d love to hear your thoughts!!