OE (Overextension) and CC (Coring Costs) are based on dev multiplied by various multipliers. The most common issues driving CC are WE (War Exhaustion) since it increases your CC by WE*3 as well as governing power.
I recommend next time you core something to hover over the button to read all the multipliers and look for the ones you can affect.
I recommend doing that for a lot of buttons in general, just to get an actual understanding of what the modifiers do.
Bad events only come if your overextension is over 100%. Do not do that unless you know what you are doing.
Imperialism CB is fine since you don't get unjustified demands on war leader provinces, which saves you dip points.
The point behind vassal feeding is to space out your mana consumption. Instead of only trading admin points for provinces you are now trading adm+dip for provinces.
Another great tip to save admin is to stop statifying everything. Way too many players are doing that. Trade Company (TC) Lands should get TC'd. Personally I also convert them before they turn into a TC.
The dip rep malus on integrations is entirely irrelevant.
Well the TC company stuff i was doing regardless. Picked religious ideas because it was a bit thematical with spain and just spending the papal influence on lots of good things.
Ill check the buttons more next time. It just seems some stuff is crazy expensive. Like for example, i was fighting acrush the revolution war, and if i took a OPM, it would have costed me 50admin + 21% warscore. Forjust a mountain province.