Should I do a day trip from Split to Plitvice or stay the night near the park?

No, two hours at the park is a waste of time. Don't bother. You'll want about four hours minimum, preferably 6-7 hours to see it without rushing through at a made pace.

FYI, there is a later bus back to Zadar and you should be able to connect to another bus back to Split. Probably not MUCH later but might buy you another 2-3 hours.

Consider also renting a car just for the day. It's an easy drive to Plitvice. Try to leave very early in the morning because by mid-morning the park gets crowded with tour groups. If you rent a car for a day, you can probably drop it back at the rental car place after hours and just drop the key - I've done that more than once.

If you decide not to do Plitvice, consider Krka National park instead. It's a little closer.

There should also be day excursion options to either of the parks, but I wouldn't expect them to be cheap. You don't a guide to explore the parks, though.

You're contemplating sitting on a bus for 8+ hours round-trip for 2 hours at the park, which is likely to be crowded when you get there? That sounds like a really bad idea. Do yourself a favor and stay overnight so you get get to the park when it opens and spend as much time as you'd like there.

And if you choose to stay a night, traveling by bus, consider staying at one of the overpriced but convenient park hotels. You could explore the park over two days - and afternoon and a morning. If you stay at one of the park hotels, they can stamp your park ticket from the previous day to enter again the second day without paying again. Close to the bus stop at entrance 2, too.

2 hours is not enough time at all. Stay the night so that you can experience the magnificent park with plenty of time. There are plenty of hotels in the park and near the park. We stayed at Ethno Houses and enjoyed it. Very cool rooms and great food. Only caveat is that they do not take credit cards.

You do not want to be in the park during day-tripping hours. It is a madhouse with crowds packed on the boardwalks. Don't try to make a day-trip to Plitvice from Split or Zagreb.

Thank you so much for your advice!!