I don’t know the answer to if City Hall will be open before the park open time, but I do know that typically they start letting guests into the park ~15-30 min. before park opening time. You’ll want to be at the gates an hour before opening, and allow for time to get through security. If opening is 8am (and you want to arrive a the gates by 7am) then 5-10 min. should be enough for security but if any later, add another 5-10 min for each hour later. Also factor in any time you need to walk (if driving you will need to park in Mickey & Friends parking garage which opens 90 min. before park opening- the other lot Toy Story Lot will not open until 60 min. before park opening and will put you about 15 min. behind which I normally wouldn’t say is a big deal, but with the chance that you need to be among the first at guest services, I would say this matters).
Then at the turnstiles you’ll be able to be one of the first in line & you can use that time to apply sunscreen if needed, eat bfast on the go, review plans for the day, and I also will even sometimes do my daughters hair while waiting. Once Cast Members start coming out to the turnstiles, they’ll scan passes for the first few groups in each line so that when they start letting guests in the first few groups can just roll on through the turnstiles so you’ll be able to enter without any delay & head right to guest services.
Hopefully this helps. Also, I don’t know if you had considered starting in DCA but I have heard that their Guest Relations at the front (Chamber of Commerce) is far less busy (but I still don’t know if they are up & ready to start assisting guests right as they allow guests to enter).