The closest is Iraq, and it's not a functioning democracy but a hybrid regime.
Blowback is going to be the biggest issue here. Ali Khamenei wasn't just the leader of Iran, he was well respected for Shia muslims. While not perfectly analogous, it's close to killing the pope.
Maybe this leads to open revolt which might fully topple to government, that said, I don't think there's a US/Israel endorsed leader or goal for succession here.
For as much as the far right in the US likes to criticize Iran, ultimately their only real complaint about their ‘theocracy with democratic characteristics’ is that it isn’t Christian.
Iran's system of government doesn't really directly relate to their geopolitical strategy. You can have the most awesome democratic system at home and commit absolutely evil atrocities abroad. Just ask Kissinger (or maybe Khamenei can ask him now?).
"is the de facto head of state and the highest political and religious authority of Iran (above the president). The armed forces, judiciary, state radio and television, and other key government organizations such as the Guardian Council and Expediency Discernment Council are subject to the supreme leader."
by every definition of democracy, Iran is very close to the worst end of the scale - just as an example https://worldpopulationreview.com/country-rankings/democracy...
Their government is structured to resist this. There are another 20K Mullah's that succeed in their place. I have no idea how much experience they have or how that works in detail. Most of them at the moment AFAIK are in underground bunkers. I'm waiting for news on those.
The council (50+ members) may elect another leader in his stead, provided the current council can hold on to their seats, which depends on a lot of factors.
> If it falls, will the democracy kick in or a civil war
If the previous Western policy for the region is any indicator, they'd prefer a monarch over democracy. Probably even civil war over democracy, but ¯\_(ツ)_/¯