Two and a half days to get from Dubrovnik to Venice and no direct transportation route.

https://www.ricksteves.com/travel-help/plan-your-trip-with-our-expert-consultants

Perhaps avail yourself of the 50% discount as a tour member, spend $40 to get a solution or 2 that work

Why not fly to Rome and then take a train to Venice?

I was checking the ferries and the ones from Rovinj and Poreč to Venice don’t start till May. Flying might be your best bet.

You’re probably best off just buying a flight ticket from Dubrovnik to Venice. There are no direct flights from Dubrovnik to Venice so it will take several hours to get there but at this late date, it may be the best option. Alternatively, you can check Skyscanner.com for flights from Dubrovnik to Rome. Vueling Airlines has several direct flights for less than $50.
From Rome you’d either fly or take a fast (Frecciarosa) train to Venice. Check Italotreno.it
Good luck!

If you fly, check connections to Bologna and Verona too, they are both a short train away from Venice.

Years ago we did an overnight ferry from Dubrovnik to Rijeka, then took a bus to Trieste, spent the night there, and then took a train into Venice in the morning.

I don't know if that ferry is still running or what the schedule/price would be. We did get a cabin, for a little sleep. We rather enjoyed the trip . . . we had a couple of hours in Rijeka, then a comfortable, scenic bus to Trieste, and a nice evening there.

Unfortunately, the Dubrovnik to Rijeka ferry doesn't run anymore. A big pity because it was a lovely way to travel.

Alternative would be the ferry from Dubrovnik to Bari (if it runs on your days, it is not daily IIRC), where you could spend the night, then long train ride to Venice.

I’m no help, but filing this in my mind to be sure if I ever do B2B tours, I figure out prior how to get from one to the other!

Thank you to you all. I'm flying via Vienna to Venice. Another adventure!