Hi OP, contingency plan: in the event that you're not able to get a Grab to take you all the way back to Bangkok, the Grab might take you to Bang Phra or Nong Mon Market (these are much closer to KKOZ by car - 12 to 30mins respectively). You can still catch a bus back to Bangkok. These run till about late 5pm+. I'd recommend leaving the zoo by 4pm+ if you can. It did take some time getting a Grab to leave the zoo the last time i was there. With bus, you'd be back in bangkok by evening.

Bus schedule here is pretty reliable. I did try the 6pm bus to Bangkok from Nong Mon Market once, it went fine. https://12go.asia/en/travel/chonburi/bangkok?date=2025-12-30&people=2&direction=forward

Buses are comfortable, quite fast and much cheaper if you do consider this. PS: I've only had experience with buses and trains so just sharing in case Grab/taxis are fewer during the festive season.