Traveling from Venice to Split and Dubrovnik. Any ferries from Venice round trip?

Ferries from Venice arrive at Rovinj and Pula, in the northern area of Croatia called Istria. You could then make your way to Split, but this might be more time consuming than you wish.
1) How much time do you have?
2) Are you open to car rental or do you prefer to use public transport?
Alternatively, you could take a train from Venice to Ancona, and get a ferry from there to Split.
Unless you have a lot of time, flying would be my preference. Check the website Skyscanner for options.
Split and Dubrovnik are four hours apart by frequent bus, or you can take a catamaran operated by the company Krilo.

There are no direct ferries connecting those cities. The main ferry routes (both ways, not operated every day) are Split-Ancona, Dubrovnik-Bari, and in summer, Rovinj-Venice or Pula-Venice; www.aferry.com is one search engine. Flying at least one way is a good idea; try www.skyscanner.com. Volotea Airline flies Dubrovnik-Venice, again not every day, or you'll usually find more flights to/from Rome. Trains within Italy are faster and more frequent than within Croatia.

Travel by train from Venice to Ancona and then get on one of the overnight ferries from there to Split. Check out train timetables – and ticket booking – on the Trenitalia website; train journey time is around 4 hours, including a change in Bologna, or there are also a couple of direct trains a day between Venice and Ancona that take 3 hours 30 minutes.

There are several companies that run ferry services between Ancona and Split, and the journey time is 10/11 hours.

I suggest flying to Split from Venice will be your best bet.

We are 76 and 85 andhave traveled independently for decades , always with a Rick Steve's book. We have been to Ancona, that would be easy for us. How easy would it be to go from the Venice airport to the port to go to Istria.

Best to check the schedules. I know of https://venezialines.com/ for the Venice to Istria line. There may be others.

Venezia Lines catamarans to Istria (Porec, Rovinj and Pula) depart from the S. Basilio ferry terminal, not from the cruise ship port. From the airport, you would need to take a bus or taxi to Piazalle Roma and then a vaporetto to the S. Basilio stop (#2 line, I think). It's a long way from Istria to Split and public transit options aren't great, so I'm not sure that would be a good option for your visit to Split and Dubrovnik. If you decide to do an overnight ferry from Ancona to Split, I highly recommend a cabin with shower. Personally, I would fly both ways if possible.

There are a few other ways to do this. Ferry to Pula evening time and a direct flight to Zadar in morning, scenic bus to Split. Catamaran in morning from Pula to Zadar is also good with island stops on the way.
On way back you could fly direct from Dubrovnik to Venice, or do all of journey in reverse.