Vance, I like what you've done with this tree very much. I think it would look great on a slab... Eventually. I don't think it's ready yet.
You know more about how Mugo grow and their repotting work than I, so I'll leave all that up to to. I'm just going to comment on styling
The issue I see is it's potted too high. Like on top of a mountain. I would prefer to see the branches laying just above the soil to be more IN the soil. So that it's not a "quasi-raft" it IS a raft. What I'd like to see is the branches that are now just on top of the soil half buried in the soil. They would become a nebari of sort.
So, I think to make that happen it first needs to be planted on a deeper pot for a couple years to develop roots higher, so you can cut the roots on the bottom off. Thus lowering the entire composition. THEN, put it on a slab.
Regardless, you have made a nice composition out of difficult material!