I've been to the D23 Expo twice, in 2017 and 2019. My biggest tip for avoiding the line to get in the building (and for any large convention) is: unless there's some reason you desperately NEED to be there right when it opens (i.e., a panel or something) wait for a couple of hours after they open before heading over the convention center. My friend and I tended to roll up to the convention center anywhere between 11:00 and 1:00, and we were always able to stroll right in. That leaves plenty of time to spend at the expo without having to start the day by standing in a massive line just to get in the door.
Lines in general are unavoidable, but the time spent in lines varies depending on what you want to see and do. Activities on the Expo floor tend to have pretty minimal lines, unless it's something that's a huge crowd draw (like the 2017 experience where you were able to jump into Scrooge McDuck's money bin. There was consistently a 2-4 hour wait for that one, because pretty much everyone at the Expo wanted a chance to do it). But just walking around, taking everything in is pretty low-stress, and it's a blast walking around seeing all of the great costumes and talking to people about your mutual favorite Disney media.
Also, a good way to avoid the food lines is to eat meals outside of the convention center. Have a good breakfast before heading out, and go somewhere nearby for lunch. There's a Starbuck's in the Hilton, and some restaurants and cafes adjacent to the Marriott. They'll still probably be crowded, but not as bad as the commissary in the convention center will be.
One thing that D23 seems to always struggle with (and I don't know if they've improved as of their last expo or not) is organization. Getting into panels has indeed been a cluster, even when they implemented the online reservation system in 2019. My friend and I had reserved a spot in one of the panels, and when we went to the basement where we were told to line up, nobody on staff knew where we were supposed to go. There was confusion as to which was the reserved line, or which was the standby line, or which line was for which panel. Everyone was given a specific wristband depending on the panel and reservation status, but none of the staff seemed to know what each wristband meant, and they were talking back and forth with each other, trying to figure it out, while people were shuffled from place to place. Even with our reservation, we ended up waiting on the basement floor for an hour before being let into the panel (which, due to the confusion, started about half an hour late). I REALLY hope they've improved on that since the last time I've been, but I doubt it. In general, I don't prioritize panels for that reason. If I get into one or two small panels the whole weekend, I consider it a win.
I'm both excited and nervous to go this year. I'm really hoping they've gotten more organized and ironed out their panel reservation and line management system. But even if they haven't, I've found that it's still a great time as long as you manage your expectations (You won't be able to do everything, and you may not be able to buy the exclusive merchandise item you were hoping for, and that's okay) and go with the flow. And it will be interesting to see if spreading the event out to the Honda Center will help with crowding, or just be a massive pain. Hopefully it will be the former!
