In General many International lines on the Balkan are suspended (due track renovation or “Covid”)

to get informations about train routes down there i recommend the website of the Local railcompanies Usually i use Bulgarian Rail and Serbian Rail as with these both websites i´ve get the Informations on most International train routes lets start with the Routes

Belgrade - Budapest (closed since 2 years for track renovation should be back in service this autumn)

Belgrade - Nis - Sofia/Skopje/Thessaloniki (not in service since spring this year The direct trains usually operated just in summer to Sofia was still a Route with two changes on the way in off -season but Belgrade - Nis - Skopje - Thessaloniki was only operated in summer season without any off season rail services )

Bucharest /Sofia - Istanbul not in service due covid The direct coach from Bucharest usually only operates till september  Sofia - Istanbul usually daily :/

Bucharest - Sofia - Thessaloniki suspended due Covid :/
Bucharest - Thessaloniki just once per week as Nighttrain in summerseason (the only train that really reach Thessaloniki )

Istanbul - Thessaloniki out of service since many many years :D

Trains to “Thessaloniki” there are usually 3Trains to Thessaloniki Belgrade - Nis - Skopje - Thessaloniki (as Nighttrain) , Sofia - Kulata - Strimmon - Thessaloniki (as daytrain) and Bucharest - Sofia - Kulata - Strimmon - Thessaloniki (as Nighttrain)

Only the Bucharest - Thesaloniki train really reach Thessaloniki on the other two Routes you have since years Bus replacement on the Belgrade train from the Greekborder to Thessaloniki and on the Sofia daytrain between Kulata und Strimmon  :/

this means you have to take quite many buses actual as most International Routes are still suspended :/