Another thing you can do using diplomacy tab is to change diplomacy status to RoW or Share Maps status with all AI on the map
final thing you can do to improve Global popularity is 100's of micro trades on stock market each day, buying or selling 1 unit at a time... or you can trade 5k silver for 5k silver with coalition mates, number of trades per day not the amount or the resource is what counts.
See these two I tested quite a bit and can't really confirm. Especially the market and I did indeed test triple-digit amounts of trades in a day for sure. I saw no short- or long-term effect on popularity.
And while RoW and Shared Maps (both to the exact same amount, too) do have an effect, it only seems plausible for this to become a factor in maps on which there are ideally several dozens of AI countries to do this with are available. The effect is also fairly minor and gets outweighed by negative influences very easily from what I've observed at least.
Global popularity, is relative to other countries in your map, elite players use it to gain RoW with AI nations early on in the map, GP is only important prior to declaring any wars, with first war it goes out the window. Since it is relative, there is no concrete % at which AI will trade embargo or declare war on you however, a good indicator is being one of 5 most dreaded in the newspaper.
And that, hnnnnnnnnnaaaahhhhh.... can't say this aligns quite well with my observations either, except for "with the first war it goes out the window" - that's certainly true as stated above and will make it all but impossible for practical purposes to attain peace with an AI since you evidently already are in at least one war if that's what you want. Unless obviously it's the AI left by a former player who declared on you, but even then... a few days of regular gameplay, which includes warfare as its major content, will tank your popularity enough. So you're absolutely right, GP is only important very early on and anything later in the game is damage control at best.
However, you can spend literally an entire game in the Top 5 Most Dreaded and recieve trade embargoes at most. Where I have found it to become dangerous is once your popularity goes into the single digits. Anything below 10% or maybe a little earlier than that already and AI will start declaring on you left and right very indiscriminately. I suspect that up until that point, your average morale and the military strength that gets published in the DE play quite a role as well.
So the way I play it is whenever there's a chance to possibly better it, like giving ROW, I do it, but I do it assuming it will not change the run of the game in any meaningful way. I keep an eye on my popularity to be ready once they start bumrushing me, but that's about it. At that point, you would usually be strong enough anyway for them to be not more than a slight bother.
