Hmmmm, a quick search tells me that the “fastest” way is via Italy. Especially if you want to get the most out of your pass. The ferry from Patras to Bari, and then work your way up to Ljubljana.

I think there is also a bus from Thesaloniki, it was mentioned here a couple of months ago on the forum.  I havent done that part of Europe.

Perhaps you should dive in the search history here? I think you’ll definitely find a lead that will help you. Anyway, rail connections between Greece-Romania-Bulgaria-Serbia are not the best. A lot of people recommend long distance buses if you travel between those countries.

In the meantime I have found a long distance bus that goes through Sofia to Ljubljana which costs +/- €170 for two people (I’m travelling with a friend). 
We came from Corfu and before that Italy (through Bari) so if it’s not necessary we don’t want to go back to Italy.

I think somewhere I also found the option to go back to Igoumenitsa and take a ferry to Venice. Do you know if the connection from Venice to Ljubljana is better and if that would be a cheaper option than that long distance bus?

Yeah, returning the same way is a bit meh. I get you ;)

Venice Ljubljana is much better doable yes! You’ll have to switch in Villach, Austria though. This summer there were construction works at the border, with rail replacement busses, but I think they finished by now.

170 euro for Sofia-Ljubljana sounds very expensive (even for two persons).
Not long ago I did this trip for one person for around 30 euro… (in 2022).
I’d recommend you taking bus Saloniki-Sofia and then Sofia-Ljubljana bus. By train is unfortunately not very convenient in this area (or not possible at all these days) :/.

From Zagreb its quite doable again by train-which you can easily check yourself on any planner.

Also: have you been to the busstation or are you one of those who believe this world is now all on the www? It is not in these parts of the world  And above advice to try via Sofia is indeed quite good to lower the fare-and see a bit more. From there you could-if you have enough time+traveldays left and like to spend that on very slow/old trains, go by train too-via ROmania. ALL trains over borders in what once was Yugoslavija have been cancelled-except HR-SI (HRvatska-SlovenIja). Budapest would also be  avery good city to start training again and should have plenty of buses to of.

I don't think customer service gives timetable advice. That's typically something you have to do yourself. Or you can pay a ravel agent to do it for you...