It's been a few years since I've done NYE at WDW but they do NOT do Fantasy In The Sky twice per night on 12/30 and 12/31. Those fireworks are only done at midnight. They have regular fireworks earlier in the night so families with kids that can't stay up late can enjoy a celebration too. I would assume that the scheduled 6:30 showing will actually be HEA as I seem to recall that the Christmas fireworks end earlier in the week.
I have been to around 20 NYE celebrations at WDW. We would (nearly) always do the MK fireworks on 12/30. It is, IMO, the absolutely most amazing not to be missed fireworks that WDW has ever produced. GO! Be aware that the Hub is usually packed to the gills, even more than usual. We booked the late dessert party a few years ago and it was soooo nice to have the dedicated space to ourselves. Back in the day, the park would remain open until 3:00 am and we would regularly close it.
We would (nearly) always do the Epcot fireworks on 12/31. We did the Studios fireworks once on NYE and it was disappointing. The World Showcase is also full. People stake out their spots pretty early but we never did. We would find a spot around 10:30ish and we would be a few groups back.
Here are the Park Closing Phases:
https://allears.net/walt-disney-world/wdw-planning/disney-world-park-closing-stages/
A lot of people tell you to go early and stay all day because the parks can and do close (at least they did pre-Covid). We always went into the park (MK on 12/30 and Epcot on 12/31) during the day, went back to our resort to rest, and then went back into the park around 5:30 - 6:00. We also always had a dinner reservation on 12/31 because the lines for food are crazy. In any case, we never, ever stayed in the park the whole day.
For people staying offsite, it can be more difficult. You must be at the MK pretty early as the parking lot often closes by mid morning. You can ask at the parking gate what you should do if you leave and come back. It may be that you will not be allowed to park at the MK again. You may have to park at Epcot and take the monorail to the MK. If you get to the MK too late and the parking lot is already closed, you may or may not be able to take the monorail from Epcot. I recall CMs scanning MBs to ensure that the people wanting to ride the monorail were eligible to enter the MK. One of the first park closures is to offsite guests who are entering the park for the first time.