If you pour a concrete slab for your floor, that will give you a lot of mass, similar to the water barrels, but it won't get in your way.

The light-blocking effect of multiple layers is really only a factor if you're trying to growing food crops like tomatoes to completion. You'll never see a big commercial tomato-growing operations have multiple layers. But with plant starts, they don't seem to mind. I have one layer of shelving, but find myself using the gravel ground underneath the shelves quite often, especially when it gets hot. Plant starts for cool-weather crops especially don't like blazing hot sun. The shade from the top layer of plants helps them not get too hot.