If I participate (because I have a lot on my plate already) then may as well take Austria-Hungary.
I'm Hungarian so I do know what to do with them.

Speaking of which, exactly what you mean by "Austria" or "Hungary" because neither of these nations were the same as they are right now.
Also nationalism was an issue but exactly for this reason as the "victors" they'd throttle it down with vengeance.
Similarly, what Czechoslovakia are you talking about? Their territory was pretty much shared between Austria and Hungary until nationalist took the opportunity after WW1.
Either way, internal politics will be handled well because by this period the current king is Charles IV who was a pretty level-headed and amicable ruler (unfortunately he took the throne during the wrong time, well not in this RP).

BTW, why did this war last for 5 years without the help of either the Brits or the USA?
That makes no sense. At its absolute best the war would've ended in 1916 without them, especially if the Germans are allowed free pass through Belgium.
Then again, why would UK allow that? The Great War was all about re-shuffling the control over colonies which the Central Powers missed out on before.