I'm playing as Milan and my game plan was to be a peaceful nation and unifying Italy without wars. However, I have no idea how to do this efficiently. Since Milan starts with 2 unlawful provinces, the first thing I did was to release these as vassals. Some 20 years later, I figured I could now diplomatically annex them without the HRE intervening, but to my surprise this doesn't seem to be the case. I still lack them as cores, and there's no way for me to hold them for 50 years without the HRE demanding that I release them again.
I've also read that claiming thrones is supposed to be key to peaceful expansions. I thought at first that this would be possible when a regency council was ruling the nation, but this was wrong, since apparently there's still a heir, only too young for the moment. Waiting for a nation to lack a legal heir doesn't appear to happen very often, so this doesn't seem to me to be a very efficient way of acquiring provinces, either.
I've also tried royal marriages and alliances with weaker nations, helping them out whenever I can, while making sure my relations stay constantly at 200. Even if this is done for what seems like an eternity, they still won't agree to become my vassals, and even if they did, I'd still lack cores on their provinces.
I know having a high prestige is also important for other nations to agree to my demands, but it seems impossible to keep any substantial amounts of it unless you participate in wars. Additionally, leaving the HRE would make it even harder.
Thus, it seems to me that the only way to do it is to leave the HRE so that they won't intervene, and then diplomatically annex my vassals. But then I'd also have the problem of making them my vassals in the first place, which I assume could only really be done through wars, as it seems rather hopeless to have them agree to it otherwise. Either that or you need to have cores on their provinces, which afaik can only be gained through popups, and of course waiting for these to appear on the proper provinces is nearly impossible.
So how do people go about doing peaceful expansions? What's the best way to do it?