Travel from Split to Plitvice Lakes National Park?

Plitvice is easiest by rental car, as ideally you need to be there as close to opening time as possible to avoid the crowds. The drive is c 3 hours, so it would be quite a long day and much longer by bus. Driving is easy in Croatia.

Direct buses take 4-6 hours each way, so you'd probably want to stay overnight at a hotel inside the park or a B&B just outside. Try https://getbybus.com/en/ and look for the express buses operated by PKN. Buses going further can be slower and make more stops.

You might consider Krka instead--it is similar and would take less travel time. I adore Plitvice, but I would definitely want to maximize time. You will return! (I have, three times since my honeymoon) You may want to have three nights on an island (from your other question).

The best time in Plitvice Lakes National Park was late afternoon and early morning. If you try to make it a day trip, you will be there mid-day with the crowds. I think you will enjoy three nights and two days in Split. I wouldn't spend one of them on a long, long driving day unless you really prefer that. We saved Plitvice as an overnight between our stays in Rovinj and Split.

Are you going to Zagreb? I recommend doing so - it's different than the coast, but very nice, although small for a capital city. If you are going to Zagreb, rent a car and visit Plitvice Lakes on the way. We stayed overnight in a sobe, and got to the park (on Croatian National Day, we found out) at 8 AM. We quickly decided that following the crowds was dumb. Most people take a bus to the high point and walk down. We walked up to the high point, and took the bus down - counter-traffic.