I would suggest we are going to have two functional football stadiums for much longer than 6 months. Everton dont immediately divest of Goodison and its expenses when they move into BMD, it will take a while with that project as well. Having the build finished is going to cost the same no matter when its moved into, its just the costs to do the move, and costs to maintain.
Instead I would think its much more costly to manage a midseason move from one stadium to another, and all of the headache and potential for disaster that can come with that. I fully understand the desire to be in the stadium as soon as possible, but it seems to me the far better solution is to give yourself an offseason to prep the new stadium for a glorious, clean, fully sponsor filled opening, clear of logistical nightmares, rather than trying to force it midseason and then having our first matches at the new stadium part of a botched opening and horrible memories. This is far too important to the club to get wrong.