Best way to create rooms with interior walls?

My floor plan was rooms around a central area (stairs, closet, laundry room). So if I try to split the floor and define, say, the kitchen I can’t because one side of the room is an interior wall.

Whats the best way to do that?

Connect the inner/island area to the outer wall on one side (shut a door or use some other obstacle it can’t see through) when mapping.

For example, with caveats…

Hitting the “Vacuum” button without selecting any room(s) will do a full vacuum, if it can finish and re-dock without issues it will update the map with what it found. Not sure what effect that will have once you set those splits. Wonder if that would solve the phantom wall issue, assuming the splits stay.