I think timing for this requires a bit more thought. What are your main goals for this trip? Is it a once in a lifetime trip? Is it a we're-in-the-area-and-adding-this-on trip so it's not the main headliner of the trip? Is it a rare troop that won't happen again for many years? Does your income allow you to easily to bump your trip up by a year? (Obviously no need to answer that last one; just something to consider!) How sad would you be at missing some rides or is this a ride-centered trip? What are you most excited about With the trip?
Personally, I'm considering going shortly after SW:GE opens. I have NO desire to see it. When they opened Cars Land 5 years ago, everyone was so focused on it that the reports were mentioning how quiet the rest of the parks were. Sounds good to me
If it's a once in a lifetime trip, or even a not-going-to-happen-again-in-less-than-several-years trip, I'd consider going in March or April 2019--most everything will be open and pretty, freshly refurbished and ready for the flood of people and press that will arrive in the days before the opening.
If you tend to be ride-centric in your trip, January is tough as they are getting the holiday overlays of some rides, and January and February are when Splash and Grizzly are down for their yearly refurbishment. Hours are less. Entertainment is less (weekend only). However, I'll openly point out that, despite those issues, it's when I do most of my solo trips.
