This, really this.
It's amazing how many people assume that "reforming" is just an automatic process; major overhauls of a system that's working just fine carry alot of risks for little tangible gain.
Even overhaul of the system which is NOT working fine can be a major problem: look at the Ottoman attempts to reform their army (IIRC, at least one sultan ended up being killed).
Now, having a strong Hungarian buffer and a weak Austria
Couple thoughts:
(a) The Hungarian buffer could be created but its strength is a completely different story. In OTL Tekeli had been routinely beaten (seems to be a decent subordinated cavalry general): it seems that the country (or whoever and whatever was in charge of raising money, etc.) was quite unenthusiastic about having a strong ruler with a powerful military force. Also keep in mind that while all that brouhaha at Vienna had been happening the Lithuanian army had been successfully looting Transylvania. Then, I'm not sure that capture of Vienna would automatically mean a complete Ottoman conquest of Hungary. In OTL the agreed upon plan (which Kara Mustafa abandoned) was to spend at least one campaigning season besieging and taking Hungarian castles. As an illustration of the general state of the affairs, Kara Mustafa did not even bother to inform Sultan about his change of plans until it was too late.
(b) Capture of Vienna would not automatically mean "weak Austria": the field army would remain intact and actually even without Sobieski Charles of Lorrain would be able to do a lot of damage to the Ottomans while they are storming the city.