With a pod after 1521, how can we get the Archduchy of Austria become majority Protestant? Would the local Hapsburgs decide to convert or was there an otl Protestant house to take their place? Would they remain united with Bohemia or were the Hussites too different to reconcile and they still try to separate under denominational differences? Would this Protestant Austria still rule Austria or would control move to the North?
The problem is not getting Austria itself to convert, but the Habsburgs to have political reasons to go with it. Which is really much harder than it seems. Otherwise just drive them away in rebellion and voila, it is done.
The Bohemian domains were linked to Hungary and fell to the Austrians because of pretty specific events in the Balkans we are even discussing in
another thread, so that's something the Habsburgs may well be interested in, but is ultimately given or denied by butterflies more than the topic at hand (save for a specific case I will mention later).
So, about political matters. Habsburgs have always cared about their own aggrandizement, pursued in two ways. One was dynastic ties to European royalty; which gave them lottery tickets everywhere, but mostly in places that ended up Catholic, giving them a lot of reasons to lean anti-Protestant and potential ties to people who wanted them to stay Catholic.
The other was that of the Empire. The Habsburgs inherited an Empire that lagged behind others in terms of relative centralization; they tried to remedy that, though mostly in hopes of extending their influence, but they could not really cow their opponents before the religious crisis hit, leaving them forced to side with Catholicism if only because their opponents were Protestants. It didn't help that by 1521 Charles V is already HREmperor, cementing the Hasburgs on the Catholic side unless he radically changes and/or dies on the spot.
Which leaves the situation of 1521 pretty complicated for achieving the triple hammy of Austria protestant and imperially strong; but if you can do that, you probably need a slightly different and more quickly successful Protestantism; especially if the Pope gets abducted or conquered by the French or Ottomans, creating a perfect scenario to redefine the Empire without the Pope and with dangerous/immediate enemies the Emperor can rally people against (instead of having to fight those same people in devastating internal strife) while pushing for more centralization.
Once they do, they're likely to campaign against Bohemia anyways; keep in mind we're before Westphalia, so there's one Church and schismatics are to be brought back, at swordpoint if necessary, something Lutherans definitely shared with Catholics and contributed to the tensions leading to the 30 Years War.