>So I just finally got into Sushi High Roller (for anyone wondering how to get in, I got in right after finishing the post-game "Looker Bureau" missions)
So, Fizz is partially correct, I guess.
You have to complete the Looker Bureau Missions which are only accessible post-game, which you can read more about here.
Once you have accomplished them, you have access to the Sushi High Roller.
Hope I helped. :)