I'll address grit first - yes to grit, it's cheap insurance and a single bag will last you until eternity (I'm still using up the single scoop of chick grit I first bought 8 years ago) so no reason not to supply it even if you have a decent amount of rocks in the soil.
That's a very small coop for the number of birds you have. There are threads that show his to turn the whole thing into a coop with better ventilation. I would do that as soon as you've constructed the much larger run.
I'll just be preemptive, though to add to what's below, you'd ideally want to build up a matching roof over the run area if going with the instructions below:
To turn it from 2 small "boxes" (tiny coop above tiny run) into 1 bigger "box" you'll want to remove as much of the inside coop wall as possible, plus the floor. Take out the old roosts too.
Nests might be able to stay as is, or may need to be relocated elsewhere or replaced - depends on the structure of the coop and how things inside stack up once done.
Run a new roost(s) lengthwise or widthwise across the newly open space, depending on how much roost is needed. Ideally you’d like 12” per bird but 10” can suffice in many cases.
Board up some of the external wire walls so that the roost area is protected from winds and rain. Do NOT fully cover up all the wire, you need ventilation and natural light, so at the very least a few inches under the roofline should remain open. If your climate allows for it, you can leave entire walls open with just the mesh, or make it convertible for the season by covering up open walls for winter, and then uncovering for summer.
Example of a modified prefab:
https://www.backyardchickens.com/threads/my-renovated-prefab-coop.1440258/
