Athens to Vienna Transportation Question (Traveling with Parents)

Hi all! I'm traveling with my parents in October from Athens, Greece to Vienna, Austria. We plan to go to the following cities along the way (but are flexible!):

• Athens, Greece
• Thessaloniki, Greece
• Sofia, Bulgaria
• Sibiu/Timișoara, Romania
• Bratislava, Slovakia and/or Vienna, Austria

I used to live in Sofia so I know the city and area quite well. We aren't 100% committed to any of the cities besides Athens (where we start) and Vienna (our plan is to catch a train there to take us back to Hamburg, Germany...so we COULD catch a train elsewhere as long as it gets us to Hamburg).

I live in Germany and am quite proficient in travel around Europe...but we're really struggling with how to get from Athens to Sofia. Caveat: my dad doesn't fly! We can't seem to find any (legit) car rental companies that will let us take a car out of Greece...so we thought to get a private driver in Greece to take us to Thessaloniki, and then a private driver from Thessaloniki to Sofia?! Other options we are overlooking?! Can't really find any trains (Athens to Thessaloniki train schedule only goes until the end of August and not sure how much my 70+ year old parents will like bus travel). Any and all suggestions are welcome or something I'm overlooking!