To the OP:
Think I would do the 6:15 show from the front of the Castle, if it were me. Will be crowded but, while I have not actually walked over to Tomorrowland Terrace and looked back to the Castle, the new Castle turrets may block some of the stage view (they are pretty big) and also there are a lot more trees in the HUB and side walkway areas over toward the outer walkway that may get in the way.
Again, I have not looked from that location but just guessing from what I have seen from the area of the Tomorrowland bridge / outer walkway spot by the new Gazebo. Was working Fireworks in that area a few nights ago and we were encouraging guests to go out into the street because of the view with the trees in the way. I'm not currently in Orlando and won't be back working until after Christmas or I would go in and take a look. Will ask my daughter to look if she is in MK sometime in the next couple of weeks, curious myself now.
Just my $.02