It depends on the line. What is your timetable?

So my plan was to take the train from lovinac at 17:09 and then change trains in Zagreb I will arrive there at 20:57 and my connection departs around 21:22, could be a little later.
Is this to optimistic or just fine? 

For the train to Zagreb, it varies, as far as I know. But the previous train is almost 6 hours earlier, so there's not much choice...

That is very true, I could always try to arrange something else but I would rather not ofc.
Thank your for the answer :)

According to recent reports in a German railway forum you should count in delays here… the train might also get completely cancelled in the worst case.
It is mostly a one track line from Split to Zagreb and during summer there are several summer trains coming from HU/CZ/AT, which are often having a huge delay. Means long waiting for the train in stations until the track is free to continue. 

So you should definitely have a plan b in your pocket It can work, but likelihood is far away from 100% .

Good luck and safe trip!

Yes, I think we're reading the same forum :)

Also, as soon as you think your connection is in danger, inform the train staff about it. They may be able to hold it (within certain limits, for sure).