Split to Greece can be done by first taking a ferry to Bari and then another one to Greece.

Since 2019 there are no more international trains from Bulgaria to Greece. But there are still trains from BDZ to the bulgarian border (Kulata). From there you have to take a Taxi/Bus to the nearest train station in Greece (Sidirokastro). From there are starting direct trains to Thessaloniki. The route Swilengrad / Ormenio is more complicated and I do not recommend this route. But I recommend to look in Sofia for a direct bus to Thessaloniki. It is not included, but normally in the Balkan bus tickets are very cheap.

Hey!

What you could do beside the huge detour by ferry: take a bus from Split and make a stopover in Dubrovnik, hike up the hill/mountain for the sunset (or just take the cablecar). Then take the next day a bus from Dubrovnik to Montenegro. From Montenegro you can take the wonderful railway line to Belgrade (although at the moment the daytime train is cancelled and only nighttrain available :/). From Belgrade maybe the railway line to Nis will be closed next year? If yes, then just take a direct bus from Belgrade to Sofia, should be around 20-25 euro, no discount with interrail. 
From Sofia you can get to Thessaloniki with another bus for 20-25 euro and from there you have decent train connections to Athens. 
Also well worth to go to Peloponnes and use the amazing railway line from Diakopto to Kalavrita!

Sorry, helaas makkers, but for this area=Balkan, an IR is simply NOT the best solution. many train lines do not exist or have been severely curtailed. Plus that buying tickets locally is often  cheaper as a full travelday on a pass.

Also: do not waste nearly whole days by doing this complicated train-taxi/lift/hitch-bus-then train again trips. Simply buy a ticket for a direct bus all the way-much easier, much more comfortable, much cheaper often too as the combined sum. You will find there is inevitably a much better bus connection if trains are so hard to find. Often in these areas of the world these buses are not even on www.

For that period still so far away rules may change so often its near impossible now to give accurate advice, even though I am aware many 1st time IR-users tend to book out of far anything monthes advance-that is not very wise.

There was/is still a printed timetableBOOK (near impossible to believe for young people today) that is called the EUROpean Rail timetable-new editions ev. 3 monthes, for around 20 eur from amazon. This used to be a monthly guide called COOKs. You can order old edtions for much less from their UK-office, these give all main timings-at that time for all EUR countries, incl ferries and major buslines where there are no trains. You need to be able to very carefully read all the signs on it. In general timings will not have changed that much, routes even less. This was THE bible long ago to plan an IR -trip

Another good site is seat61-it gives in /en/ all overland links from the UK to all EUR countries-from d´n Belsjiekse simply hook on on the way.