Parties can sell out at any time, so if this is the only day that works for you I would go ahead and get the get tickets plus getting them early means you can go ahead and make your fp+ selections at the 60dy mark for on site and 30dy mark for off site. Crowds depend onperspective and location. The hub will continue to be crowded, especially near show times, parades, and the fireworks. However usually ride lines are great, and the fireworks can be seen and heard all over the park, so you don't need to crowd in at the hub. Also the parade route is usually less crowded anywhere other than the hub. If your adr is in MK, I would definitely keep it. You would still have time do a fp+ 3:30-4:30 with 30 minutes before your adr, and have one 4:30-5:30 that you'd probably make it to depending on how fast your family eats, and the 3rd fp+ 5:30-6:30 would be no problem. If your adr is outside MK, depending on your family situation I would reccomend an earlier adr to take full advantage of getting into MK at 4, and doing fp+ or depending on the characters you want to see (Jack and Sally or 7 Dwarves), lining up early for them to maximize party time. Either way you'll have a blast!