Cool project, very ambitious👍🏻
It’s best practice to first blockout the entire Scene. But it’s not compulsory.
The Mirror problem, is, because these Selected Vertices form Faces, which will be interior Faces, when you Apply the Modifier. You can Delete them before Appllying the Modifier, or after Applying: Select them, X > Delete Faces, followed by X > Dissolve Edges, or X > Delete Edge Loops.
I’d indeed use Cylinders for that side table.D
