I didn’t even find the “sold out” 10/31 party we did on arrival day in 2018 to be that bad! Not sure if it was more kids trick or treating or in character lines or what but ride lines were good.
This was 10/30 (not 31) last year:
The people soup wasn’t overly thick. Daytime crowd (pre-party) was also low.
We got to walk on Splash (no line!) for one final pre-Tiana ride, after doing a back to back Big Thunder, all at prime party time (7:30-8:15ish).
In early Oct, we went into MK at 4:00 pm for our Halloween party, which started at 7:00. The crowds were fairly light. I have heard that is typical. A lot of people don't want to go to MK on a day where they can't stay till typical closing time. I think it's actually a good strategy. One word of warning... it seems like everyone learns the "great strategies" online (forums, YouTube, word of mouth, etc) and starts using them. Then, suddenly, they aren't great strategies anymore!!! It is so hard to predict with Disney. But for now, I still thinks it's a good plan.