Sorry, didn't see this. John George is hard to pin down because he was exceptionally loyal to the Habsburgs for a protestant Prince. Of course some of that is the fact that as a Lutheran he was not a big fan of the Calvinists like Frederick or Bethlen or many of the Bohemian rebels. So I was thinking of a scenario of a widespread Habsburg collapse in Bohemia, Hungary and Upper and Lower Austria after the death of Ferdinand and Leopold and the loss of Vienna to the rebels. In that scenario the Habsburgs would be hard pressed to mount immediate resistance even in Austria and so it could encourage Bethlen and the more extreme protestants. Then that could be enough for John George to see himself as a bulwark against anarchy and widespread war, not to mention Calvinist expansion. So he accepts the Bohemian crown but then tries to cut the Bohemians off from contact with Bethlen and the rebellions in Hungary and Austria and of course he supports a Habsburg, probably Albert, as Emperor. So it wouldn't be as much of a betrayal of the Habsburgs if he's not stealing Bohemia from one of them and still votes for an Archduke to succeed Matthias as Emperor.
Like I said I'm not sure how likely it is. It could also be argued that John George would still decline. Then the Bohemians turn to their number two choice, Gabor Bethlen of Transylvania, and then he might accept (OTL he also declined) because with the Habsburgs in total disarray he might see it as the opportunity to claim Hungary and Bohemia. Then John George probably cuts the same deal with the Habsburgs as OTL (Lusatia in return for aiding them in ending the Bohemian revolt). At any rate I do think that you have to consider John George and Gabor Bethlen as just as likely if not more likely than Frederick to become King of Bohemia with this POD.
One other thought I want to toss out there. Perhaps if Spain gets ambitious they could see the collapse of the Austrian branch of the family to set up one of their own in the Empire? If Albert is elected Emperor perhaps Philip III considers naming his younger son Ferdinand as heir to the Netherlands with the goal of positioning him to be elected Emperor in turn? I think the Spanish would struggle to garner much support for such a move in Germany, and it would raise the ire of the French and the Dutch for that matter. But it would be interesting to think about the Infante Fernando as sovereign in the Netherlands in succession to Albert and with Alsace too.