Perhaps, instead of throttling attacks, it is simpler to make death less of a penalty of losing "a few weeks of work in 1 minute".

What if a dead country can choose to restart as the dead country, but "revived" with 1/4 of its land/buildings/cash/military/everything with a 200 turn forced protection period? This means, killing a country would only just put it out of the picture for roughly 3 days, but can still return with a semi-decent country.

This will force war efforts to focus more on crippling countries instead, and if you were to kill a country to put it out of action, you have to make sure it comes back really small or maimed. That is, you might want to focus on nuking/grabbing/destroying away half a country's land/buildings first before actually killing it.

This will also mean killing a country with a large cash/tech reserve is less effective as it can still return with 1/4 of that cash/tech reserve. However, this could promote the use of other currently "useless" spy ops such as bomb banks, steal tech, etc, if they can be re-balanced properly for it. Make all harmful spy ops publicly available on the news just like normal attacks, so that it cannot be abused for netting resets, and that it can be properly accounted for in war stats.

With these type of changes, I think throttling wouldn't be needed. Fix the underlying problem, not the symptoms.