The general strategy was the standard Orthodox HRE with all the 1.3.2 cheese available. Making blobs release countries since they kept the cores. Having a ton of diplomats and bunching wars to save on dip with culture convert. Releasing countries at the end of a war cycle for the AE reduction. Perm, Nizhny Novogrod, and Livonian Order were my staples for a nice -210 AE. Orthodox HRE to go from being elected to revoking the privilegia in less than 45 years. Using the HRE to stay small since AE scaled with size in 1.3, to stay allied to France and off its rival list until I was big enough, and to make the 500% OE at the end of a war cycle manageable. As well as coring outpost provinces like Lika and Navarre to deal with European fighting and not having to walk all the way back to Russia every time.
The weird cores seem to be a problem unless you realize the rules. You shouldn't annex after a certain point unless you could release it within the month. Realize that core timers get cut in half if you release the country. This meant releasing countries and vassalizing the original (with truce breaks and massive OE if needed) was the way to go. Don't have a vassal for more than 50 years if you want to release it as a HRE vassal with all its cores. Keep cores alive by fighting with countries. I would often ally a country to declare war on another just to get cores to reset another 50 years. Finally, even if you do everything right, there are going to be situations where you will need to sell a vassal back its original provinces, so double and triple check the available cores when adding them to the HRE.
I didn't get any PUs (though in one of my earlier tries I PU'd a France that had already PU'd and integrated Portugal), but got lucky in other ways. I didn't have any terrible leaders and overall they were definitely above average. With some heir management, I avoided regencies altogether, some by mere months. Won 3 of the 4 'send for a medicus' events, and the one I lost, I actually got a better heir. Ming didn't explode before I got there. The Timurids did explode, but did in stages so it was fighting and resetting cores until I vassalized it at the last second. Burgundy became a decent colonizer. I was able to coax Denmark along until it integrated its PUs for easy and AE free core returns. And finally, I got a high admin ruler for when I westernized before 1550.
Setbacks were relatively minor. I was unable to secure Provence before France took it which caused me to have to integrate Castille and Portugal and have all their provinces (thus lots of AE scaling) until I could integrate France much later. I was also helping Poland along so it could form the Commonwealth (to release and feed Lithuania), but right before it reached Admin 10, Poland's 20 year old king died somewhere in Crimea (which didn't even have an army at the time) and lost the PU on Lithuania. I should've hit the colonizers asap or at least gotten them into more wars with France. GBR and Portugal got a bunch of colonies away and required a couple extra wars, OE and dip taking a lot of colonies, and a truce break to come under under control. And Castille? Listen, and understand. Castille is out there. It can't be bargained with. It can't be reasoned with. It doesn't feel pity, or remorse, or fear. And it absolutely will not stop, ever, until the world is colonized. Surrounding the capital and taking all its ports won't stop it.
I didn't think that there would be enough time to colonize everything since I released countries so late, but I underestimated their desire to spread across the globe. I also went around and put soldier on places where the AI is going to colonize to prevent natives from spoiling the AI's progress so it ended up being a complete WC. The last large war cycle took place around 1770 and I finished the WC by annexing Albania.
Ending screen + ideas
Also started as Muscovy.
