It really depends on where you are in MK. You could easily be lots of places and not even realize there are launch spots going off behind you, or you could be somewhere with a more open view all around and it’s really awesome. It was better inside MK before they reduced the launch points as the chances of more being visible from your particular location were increased, but I would personally still make the general statement that the 360 perimeter fireworks improve the show more for monorail resort guests than they serve to improve the show for most inside the MK, in my humble opinion.
One thing to caveat, while Fantasy in the Sky is nice show, at just over 10 mins it’s relatively short by today’s standards. I love it, but I wish it was longer. It’s over quickly.
I think turning in early 12/30 isn’t a terrible idea if you’re more focused on 12/31. On the other hand, when at WDW for NYE one could argue you should try to do all the special things.
Not sure where you’re staying but leaving MK after the 12/30 late show will be a bit of a process. Build that into to your expected “up late” time.
Hope that helps!