The fast Freccia trains are like airplanes. You get a ticket with a specific seat reservation. If you get off in Bologna, it is not valid on any other trains, so you would have to by a new ticket to continue on to Venice.
Here is a compromise, buy a Freccia ticket to Bologna, get off and explore, have lunch (practically obligatory in Bologna), then when you want to head to Venice, buy a Regionale ticket and ride the hourly Regionale Veloce train to Venezia S. Lucia. They are only a bit slower than the Freccia trains (they make more stops), go once per hour at 20 past the hour (give or take a couple of minutes), and the price is always the same, 12.50 EUR per person. Since they are unreserved, get to the platform 10 minutes early to make sure you get seats. The trains originate at Bologna Centrale, so they will be spotted on the platform and available for boarding 10 to 15 minutes before scheduled departure. You can buy the tickets out of a vending machine ahead of time, then you decide when you want to go, head to Bologna Centrale, validate your tickets in the platform stamping machine, and board the waiting train.