I want to share my route, any advice will be helpful because I've never been in Europe. Thank you

Budapest - Belgrade still no train since few years (You can go by train with a small bus replacement but it´s not a good one Budapest - Kelebia - Subotica - Novi Sad (arround novi sad you have a short bus replacement) - Belgrade

Belgrade - Thessaloniki still no trains due Covid (nobody knows then they return as direct services)

You may catch from Budapest a train via Vienna to Venice,Bari and catch from there a ferry to Greece

Beyond Bp=Budapest there are simply no really feasible ways anymore by train. South from Bp the main line to Kelebia=borderstop in HU is under complete renovation since a few weeks and NO trains-you can do a kind of shuffle via Szeged+bus, but rather I would use a direct bus Bp_Bg

Also NO trains at all over border RepSerb-Maced and Mac-GR. If you want to visit GR its also quite a detour and quite expensive to go via IT=boat. Today WIZZair=the cheapo airline for East-EUR has a special birthday sale=all tix 18% off. They offer many flites in this region.

Also IF you can preplan advance (=fixed booking, no go=money lost) trips from B to Pozn and P to Krak and even Kr->Bp cost less as 20€/seated. You may want to check if a pass for less days+some other tix is wiser/cheaper.

The very expensive and hard to get leg is BCN-Par DIRECT-if youre willing to take more time and changes and go by local tren till border ES/FR and then use fast TGV from Perpignan or Montpellier (or the overnight train from border stop=Hendaya) you can save a few 100 TRL.

This also applies to P->AMS; the direct trains are Thalys and cost from 35€/advance single, but the SUPPLmt for a pass is 30€+you have to use passday. Again can cut a lot by use TGV till LIlle and change-via Antwerpen=local trains, NO REServ needed

Thanks a lot, I’m making some changes now. Maybe I’ll change to Split instead of Belgrade. But I don’t understand about BCN-Paris is expensive or saving money with TGV or Kr->Bp cost less as 20€. I’ll get the global pass why am I paying more for tickets? 

You don't pay for the ticket as the Interrail pass is you ticket, but some trains, often high speed trains, require a mandatory reservation that you have to pay for.

You always need to check if there is mandatory reservation for the train you plan to take.

On some routes there are both high speed trains with mandatory reservations and regional trains without mandatory reservation.

TGV (mainly France) always require a reservation and this reservation is more expensive for an international journey than for a domestic TGV journey. This means that you, if you want to save money, can take a regional train across the border from, say Switzerland, and then board the TGV within France. Or the other way around. That you leave the TGV before you leave France and take a regional train across the border out of France. 

Ooh, okay..

Okay. What is the best way to finish my trip in Greece, after Budapesh? or anywhere, I want to end my trip in Greece, other places can change but that’s what I’m sure. WIZZair?

Budapest - Split - Zagreb - Ljubljana - Trieste - Venice - ferry to greece 

Between Split and Ancona you could take a discounted ferry. 

Now this

[Budapest > Prag > Krakow > Poznan > Berlin > Amsterdam > Paris > Milan > Rome > Bari]

{10 days within 2 months}

> Patras > Athens

looks more logical to me
What do you guys think?