Can we get off in Bologna on our way to Venice?

If you are travelling on a high speed train, which you almost certainly will from Bologna to Venice, the ticket is only valid for a specific train. On your ticket it will say date, time, train number, coach number and seat number. You must get that train or buy another ticket.
Even if you are travelling on Regional trains, once you stamp your ticket at your starting station, it is only valid for a fixed time, so it would have to be a very short visit to Bologna.
So the short answer is no, you will have to buy a second ticket.

If your ticket is to Venezia and has a seat reservation on it, you need to buy a new ticket. Whether you can split the ticket you have into two new segments depends on the type of ticket you have (super economy = no changes).

If your ticket doesn't allow a break, a more or less hourly Regionale Veloce train brings you from Bologna to Venice for around EUR 12 at a speed not much slower than the more expensive Frecce or Intercity (check at www.trenitalia.com/tcom-en).

Thank you😀