The Ottoman commander pretty much fumbled the entire Vienna business.
Not an excuse and has never been. Being able to conjure competent Staff and self correcting mechanisms in the chain of command is part of a nation's war capabilities. The French kept being ridiculed by the Spanish for almost an entire century precisely for that reason.
He slaughtered those who surrendered
Choices and consequences. There was nothing gratuitous in this move. It was either because he couldn't afford to carry them along, due to terrible logistics, or to improve his own troops morale.
he tried to siege the city despite having more than enough men to simply take it by Zerg Rush
Indeed, blindly storming the fortress and trying to win through greater numbers alone would have been his only option. And then suffer considerable losses, being trapped in Vienna and starving with no hopes of reinforcements on his side.
The Ottomans overestimated their capabilities to take a Western fortified city swiftly and with minimal losses. Constantinople defences were severely outdated while Vienna's fortifications were up to date to the latest Italians designs.
when aid finally came for the city, he divided his forces.
What he was capable of assessing from the situation. Being trapped between sallying besieged and relief army is the worse thing that can happen.
Had he not fumbled all of these points at once, Vienna would have been taken.
Had, if, could, would. Easy to rewrite history and an experienced commander's decision. At least the Ottoman didn't seem so confident that it was merely bad luck since they did not attempt a Siege before the next century. And at that point they only did so because they were certain the Emperor was too busy with his other rivals that he wouldn't have the means to deal with them.
why? it would represent people getting pissed off at the state diverting its attention (and money) to some distant, unknown land in search for still unclear riches.
still a better reason than overextension mechanics.
1. From a simulation's perspective it is simplistic, unfair and misplaced. You might as well have your every decisions cause unrest because there will always be parties dissatisfied with each decisions (Why build so much ships, why maintain such a large land army, why construct this building and not this one...). It's also worth mentioning that colonisation brought Portugal and especially Spain massive amount of resources, gold very quickly. Which isn't the case in game.
If anything in the case of Spain and England it in fact reduced unrest, as it was the perfect place to dump undesirables. Many conquistadors were jews and moors, English settlers were religious minorities). Colonization offered plenty of opportunities for the estranged elements of society, especially impoverished nobles and soldiers who would otherwise become trouble makers.
2. From a gameplay perspective you don't need a nerf to colonization at all. No reason to encourage standard circular blobbing even more than is already the case.