The easy route is via Romania and Bulgaria :
There is only one daily train entering Turkey : most carriages come from Sofia but during summer a couchette carriage is attached from Bucharest. Since this train is not available online for passholders, it might make sense to book a regular advance ticket through CFR (Romanian railways).
Otherwise expect to be flexible and wait a day or two in Bucharest before there’s availability or board regular seating carriages as far as Dimitrovgrad and then buy a couchette/sleeper from the attendant (cash only, AFAIK).
More info on seat reservations in general :
https://www.seat61.com/interrail-and-eurail-reservations.htm
There are (almost) no international trains and very few domestic trains in the Balkans. You’ll rely on buses : they’re not included but tickets are rather cheap. Timetables are rarely available online. Adventure!
All in all it might not make sense to get an Interrail/Eurail pass for this trip.