I may try to put in a quick trip report but since the previews are so much different than regular operating days, I did want to put in some quick notes as I will keep putting it off.
Stayed at Terra Luna for a few days. Hotel had some minor issues you would expect at a new hotel but they are quick fixing stuff. Only option after 10pm for food on-site is the pizza delivery (and a couple market items) which was disappointing. If you stay all the way until close at EPIC it is likely you won't make it back before 10pm if you catch a last second ride like I did (Stardust Racers).
Preview crowds were low when I went. I will say if you have a chance to go during the previews this will be the best crowd levels you will get. My first day I got to do everything in the park (except the Harry Potter World show because it was down) and that was without express pass. I arrived at about 845am and was in line about midway between the gate and the outer restroom. The gates opened right around 945am. You go through the metal detector check and then about 50 feet later they scan your ticket. They do not check hotel keys till you either get to Stardust Racers or the portal for Nintendo World.
I mentioned the attendance was low. If I were to guess they are running only about 10-20% of their anticipated attendance long term. Mostly I base that on restaurant and queue capacities since they were empty. The only time the restaurants were busy was prime lunch time and even then I would say the most crowded were 40% full. They are definitely operating on either thin margins or a loss during these rehearsals. Once the park starts selling to its actual capacity, the lines are going to be huge and frustrating.
The Harry Potter virtual line is a mess. As of this week, to access the virtual line you navigate in the App to the Ministry of Magic page. You will see a blue indicator saying it is a "virtual line" attraction on the right side. I recommend making it a "favorite" in the app so you can get to it quickly. Once the time to sign up drops, you refresh the page. Another small icon will appear on the left side of the page under the ride description that will be small text that will either say "virtual line unavailable" or I believe "virtual line" (it will not be to the second, it was usually bout a minute after). You must then select the "virtual line" when it pops up. The app will then ask you how many in your party. You submit that and then select a time and submit it. You then probably get a message that will say "oops, that time is no longer available" and it will send you back to the times. Keep submitting times until you hopefully get a blue screen of life that will give you a ticket. It is super frustrating. The same times will keep popping up even though it just told you the time was full. I submitted 24 requests before the queue closed at one point and did not get one. On my successful attempt I also had submitted over 20 times before I got one. Hopefully it gets better in the next couple weeks because in fairness they are both working on a new virtual queue and the ride itself has not been opening until 3pm. Until it opens they have been allowing people do do a walk through of the queue.
Overall, I still highly recommend going during the previews because of the capacity. The various guest services/concierge spots throughout the park are helpful. Use your own data if you can since the Universal wi-fi was spotty. Also, bring a charger since everything uses your phone. The facial recognition lockers were easy to use. Yes, there was down time for attractions which can be frustrating but as I said I still did everything (Monsters 3 times). The meet and greets are great in Dark Universe as well.
I may try to write a full write-up in trip reports, but I hope this helps people. Have a nice day.