You can hopefully get some tips on how to make your reservation here:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

If you have a look on Austrian Railpage there is a warning that this trains have a Bus replacement on the route (Between Graz & Gleisdorf) because of that you cant reserve this train :) 

at 10.56 starts a different train that takes a different route via Hodos to Budapest - Deli :) 

Thanks for that info, do you by any chance have a link because I cannot seem to find the info about the replacement train

https://kundeninformation-pv.oebb.at/dam/kundeninformation/extern/OEBB/2022/2022_07/Serv15049_Gesamt.pdf

Thanks, just wondering if there’s any detail regarding the journey after the rail replacement leg. Just slightly concerned that I might miss an appointment in Budapest

Check the Austrian timetable regulary they will add within the next weeks the timetable (usually atleast 6weeks in advance)  if they didn´t change then the Bus will try to keep the time of the regular train. 

Brilliant, thanks so much for your help :)

What time do you really HAVE to be on arr in Bp?

Assuming you have a pass: you could check times-in segments, better to use oebb.at or bahn.de (the latter=german and can use for nearly all of EUR=much better as the often faulty app from EUrail), via Wien (Maribor-Graz-Wien), from Gr all RJ trains with hourly service. On bahn you can sometimes set the route via special spots-or do that with a stopover time of like 1 or 2 mins.

AFAIK none of these trains have mandatory REServ service-and this was also the case for the direct train via Hodos-unless this has also changed very recently-I cannot keep up with each&any minor change.

Note that for these SEV-buses the times are quite often simply not right-some railways simply do not bother to change it all. The train after the busleg may wait-or not (often not if its a frequent service), this really varies an awful lot between countries. Any timings for buses should also be taken with a big spoon of salt.