First off, I question if you'll be finished by 6:30pm. That's a relatively quick meal, if you're even seated on time - which is questionable any day, even more so on very busy days like 12/31.
As for fireworks viewing, the main Poly beach area (between the boat dock and the first bungalow) is reserved for Poly guests for both fireworks shows on 12/31. We stayed at Poly the last two years - in 2017 the signs for the reserved area specifically said "only Resort Guests and Guests with Dining Reservations." Last year, the signs said "Resort Guests Only." Even though I was there, I couldn't tell you if that meant it was only for resort guests, or if dining reservation guests could also access the area, from what I saw on the ground.
I didn't think the reserved area was particularly crowded for the 6:30pm 12/31 show last year. Things are more congested outside of the reserved area with non-resort guests.
Generally speaking, the early show is less crowded all around the resort than the late show.