If you want an empty middle, stay away from exit rows or bulk heads, those getting late seat assignments will pick those rows even if middles to get the extra leg-space. You best chance, if any at all, is a row in the middle of E+.

Note Economy Basic and NRSA don't get seat assignments until closer to departure. If this is a leisure destination, weekend flights, ... There can be many EB.

Having B/C seating, means A is open and a window E+ seat is an attractive seat to many.

You best protection of a middle is lots of aisle/ windows still open in E-. But some will still ask for any E+ seating (over any E-)

IME, recently, 10% open seats in E+ is the best one can hope for.