We went to the last Epic preview day on the 19th. They are NOT ready for full opening. I know its a preview yadda yadda but only 2 of the rides that we were even able to ride were fully operational and I dont see that changing in 3 days.
There were places to buy express in the park but if it's already sold out then I wouldn't hold out much hope, but it's at least worth a try. Unless some miracle occurs between now and then I would say express is a must if Ministry, Mario Kart, and Donkey Kong are on your list. We didnt get to ride Ministry because it broke down before it even opened, opened back up hours later for about 30 minutes, then was broken the rest of the day. MK's standby line is SLLOOOOOWWW but you could see Fastpass people breezing through. DK is similar but not as slow as MK.
If you have early access, don't go to Nintendo World, go to Ministry and hope it opens on time.
Bring an umbrella, the sun was PUNISHING especially in Celestial Park.