from Venice Italy to Zagreb

You might also take a look at GoOpti, sort of a book-ahead Uber system. I used a GoOpti van from Ljubljana to Venice in 2018 and all went quite smoothly. Price was extremely reasonable. It was a shared van with 6 people in it, with pickup at my hotel in Ljub and dropoff in Piazzale Roma in Venice. GoOpti.com. Putting in a date in about three weeks, a GoOpti would take about 5 hours and cost between 49 and 61 euros, but it can be less if you are a bit flexible with pick-up times. It's extremely popular in that part of the world.

Another option would be to take a bus (or GoOpti as mentioned above) from Venice to Ljubljana and a train from there on the Zagreb. I don't know about you, but to me, six hours on a bus (or in a van) would drive me nuts - I would want a break. Ljubljana is a lovely city; you could leave bags at the train station and go explore the city on foot for a few hours (very walkable) then come back and catch the train on to Zagreb. It's a pretty train ride in parts.

We took the bus from Rijeka, Croatia to Trieste, Italy. It was very direct and inexpensive. Looks like getting from Venice to Trieste and the Rijecka to Zagreb would be easy.

But beware: Zagreb train station is where (fake) Agent Grant met up with James Bond in the movie "From Russia With Love." If you are going to meet anyone there, make sure they are trustworthy by challenging them with the code phrase, "Have you got a match?" They will respond, "I use a lighter." You will then have to challenge them again with the words, "Better still." They will come back with "Until they go bad." Safety first, at all times.