There aren’t really safe chemical fragrances to keep on litter. Scented litter is a no no for a reason. But, wood stove pellets have no smell if the box is kept clean and the buns are altered (seriously, I can put my face next to their boxes unless it needs cleaned) so a small apartment shouldn’t smell from it.
You can shave sprigs of cinnamon and dry lavender and bag it and hang it on the wall well above the litter boxes if you need to make it smell of something else. Keeping a box of baking soda (well out of bun reach) in their room can soak up the smell of when it starts to neer changed as well. Another good trick for small areas is to boil water, cinnamon and orange peels in a pot and let the steam spread (no lid). Leaves everything smelling like fresh pie all day.
Also, remember when changing out the box, wash the box itself in hot water and vinegar. The plastic smells if you don’t wash it.