Business trip in Antwerp, then 2 weeks to travel

Rick’s Train Travel Time & cost Map gives you an overview of faster train travel times in hours, as well as regular (full-price) 2nd-class fares. How to Look Up Train Schedules and Routes Online gives you the DB train schedule link for more specifics. But I think you'll also want to compare flights at www.skyscanner.com. Connections from Piran to Trieste at the Italian border are by bus.

"Flights suggestions in and out would be appreciated. "

Without knowing your home airport, no one can give specifics.

Look at Kayak Multi City: https://www.kayak.com/flights?mc=y. Put in From your home city and To Brussels as your first leg, and From Venice and To your home city as your second leg. See what comes up. If you don't mind a longer train ride to Antwerp, you can see if flying into Amsterdam instead of Brussels does.

Because of the distance and travel time between the cities you've chosen, your itinerary is indeed ambitious. You must plan on losing a full day between cities (check out, get to airport or train station, take train or flight, get to new hotel, check in).

A good way to learn about transit options between any two cities is Rome2Rio: https://www.rome2rio.com/. But don't rely on them as the final word - check directly with the company operating the flight, train, bus, etc. In particular, they do not show the advance purchase discounts available on trains. But it will start to give you an idea of how much time it will take.