It took me forever to get expert on this level too. I finally figured out that the longer I waited to advertise any of the booths, the better I did. I think I was about halfway through my time before I advertised any booths.
First I upgraded (any) booth to level 2. I upgraded 2 booths to level two before ordering any restock. Then after ordering restock I upgraded the last booth to level 2. Then I upgraded booths to level 3, ordering restock as needed.
As soon as all booths were upgraded fully I hired help and upgraded them both all the way.
Then I upgraded the car all the way to the helicopter a nd upgraded the storage as well.
As for advertising, once I was about halfway through my time I advertised 2 at a time (toys and souvenirs.) I waited a bit more after those had expired before checking what I was behond on. I advertised just that booth. Then, when there was only about a minute or minute and a half I advertised 2 at a time again (whatever I was behind on) to get the last items sold.
I had about 10 seconds left on the clock when I finished.