All my travels start in Zagreb so i've been there with both economy and business tickets. The lounge really is not worth it. Only plus is that there are not many people as the waiting area at the gates can get crowded because all the gates have one single waiting area.

For me personally i would never upgrade a short 1-2 hour flight. Only reason i do fly out with a C class ticket from Zagreb is when i have a connecting flight somewhere in Europe to Asia/US. For flights in Europe there really is no need to upgrade as the seating is almost the same for business and economy class and after you get of the plane you get on the same bus as economy passengers and passport control is same for everyone so there is nothing special just lounge access and a bit more food on the plane.


Edit: I am not even sure you can get lounge access on a domestic flight because the lounge is in the international departures area. I never flew domestic before.