Re: breakfast bar
Use 1/4" plywood. It will bend the the curve without problem. When you finish the rough opening, install blocking along the arc to nail the plywood to. For fire blocking purposes, you should also finish the drywall under the plywood and trim.
The arched face can also be handled with plywood, then either veneer the edges or install scribe molding (1/4" x 3/4 material ). Scribe mold is usually bendy enough for larger arcs.
Before you cut the hole in the wall, be certain that it is not a load bearing wall. If it is, you'll need to install a header to carry the weight
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!