Hi, I add the same problem so I made the reservation once in Norway at the central station in Oslo. As the reservation is currently not available online, you shouldn't worry too much about availability :)
I did a similar trip a few years back. From Zagreb to Split by train, Split-Mostar by bus, Mostar-Sarajevo by train (beautiful), Sarajevo-Bar by bus, Bar-Belgrade by train (also beautiful). The bus are actually quite cheap in the region so I think it worths it.
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