Travel from Split to Plitvice

I have not taken the bus, but I have been to Plitvice twice. I have seen the bus stops at the entrances on the main road. People use them. It is not a far walk from the bus stops to the park entrances. There are park-run hotels by the entrances too which would be the most convenient for someone arriving/departing by bus. The park hotels are certainly overpriced for what you get (I stayed in one on my first visit), but they are fine, and they offer you the advantage of getting your park ticket from the previous day stamped so you can enter the park again the next day without paying again. And you can walk from the hotels right into the park.

There are other non-park lodgings not too far from the entrances; sometimes the soba owners offer a shuttle service to/from the park if you don't have your own wheels.

I would highly recommend splitting up your park visit into two days: first evening, second morning, to avoid crowds midday. Of course, that depends on the bus schedules. Are you going back to Split after? Just check the schedules.

I have twice taken the bus from Zagreb, and it was fine, but never between Split and Plitvice. The one thing you need to know about Croatian buses is that (at least this was true in 2015) there's a small fee to stow a suitcase in the hold. It was about US $1 at the time of my most recent trip. So when a bus-company employee holds out his hand, it's not a scam or a push for a tip.

There's more than one company serving that route. Here are a couple of websites to check:

Bus Croatia << I'd start with this one. It shows two companies running buses.
GetByBus

I'm not convinced you can get reliable bus schedules as far out as October, but there's no harm in harvesting what information you can and then checking back periodically. As of 2015 online information about bus schedules in the Balkans was so iffy that when I stopped in at the tourist office of my depature city to ask about bus timing, they didn't go online to look for the information; they picked up the phone and telephoned the bus station! That technique is probably not going to work for a tourist, because the person at the other end of the phone may not speak much (if any) English. I always figure that in that part of Europe, the only schedule that matters is the one I see on the wall at the bus station, and even then I'm just a bit nervous. I do think it's a good idea also to consult the local tourist office, which should be aware if there's any uncertainty about the bus service.

Definitely be aware that bus schedules can vary a great deal by day of the week.

The bigger question is where are you going after Plitvice? If you are returning to Split, I would rent a car.

I wouldn't rent a car. It's $25/person/trip from Split to Plitvice on Flixbus, and the trip is 3.5 hours. If you are 2, that's $100 for the round trip. I can't imagine a car rental of $100 for the 3-4 days.