I don't.
I start with a couple inches of bedding then add another couple inches every week or do -- depending on when it seems to need it.
Once or twice a week I throw a handful of scratch into the bedding to get the chickens to stir it up. If the power of chicken labor isn't cutting it and...
It's odd. It looks like there should be enough ventilation.
I use deep bedding in the coop I linked earlier and I have to clean it every 6 weeks or so. It's almost completely odor-free.
???????
🤔
Can you get photos of your coop so that we can see your layout and, most importantly, your ventilation?
15 birds is already right up next to the maximum for an 8'x8' coop -- because if that is the exterior measurement you don't actually have the 64 square feet you'd need for 16 birds inside...