much better to use direct bus from Sofia-compare prices, as going onward to ATH may not cost much more.
Don't know about scenery from the bus window, but train ride Thessaloniki-Athens is pretty scenic on some parts of the route, it's not a major factor, but still. If look at bus options, then there is also plenty of not very expensive buses (official and semi-official) directly to Athens from Romania
IF you can preplan your train trip by OSE then prebook-and hope+pray its possible- as they have now the advance sale tactic-singles from 9 eur- or 19/29 etc. May work out cheaper as that railpass.
Well, if luticus intends to use daytime InterCity trains, then indeed 9 euro tickets will be cheaper compared even to plain seat reservation obligatory for pass holders. Although then it is a good idea to allow some extra time for change (both in Sofia and Thessaloniki), since not just train from Bucharest to Sofia can be late, but also buses from Sofia to Thessaloniki aren't always on time
That changing to GR bus from border across KUlata-well-do that only if you have seas of time.
This option was proposed more as for rail fans
Obviously if timing is an issue, then it's not worth it