Combining Plywood on a Layout - Model Railroader Magazine

It sounds like you're prepared to remove everything and start again.  Is that the case?

Think outside the rectangle.  If you're really going to start over, have you considered an around-the-room shelf layout?  (Maybe not, based on your room and "family concerns," but it's an option for many.)

How long is the layout going to be there?  If you move, will you be able to get it out?  Consider not only the physical size, but also the weight.  Plywood is heavy stuff.

Finally, have you thought of using foam instead of plywood?  It's lighter and in many cases a lot easier to work with, but it's not available everywhere.