Here are some tips:
(1) In the early game, the amount of trade income that you get is not that significant. Strategically place your merchants, but also set them to "form communities," instead of maximize profits. The improved relations with the nations that also share that node is worth much more.
(2) When you start a war, immediately go to the peace treaty interaction and select the peace that you'd like (you do start a war knowing more or less what you want out of it, right?). Check out who is going to be really upset with you after you peace out. Are there any big problems there that are not already your rival? If so, set a diplomat to improve with them while you are at war. No nation can join a coalition against you that has 50+ relations with you. Since your war will take a couple years, that's several months if not a year or two of work your diplomats could be doing beforehand.
(3) Prioritize Prestige -- Prestige is under-rated by most players, higher prestige helps your AE evaporate faster.
(4) dismantling coalitions, part one -- you can fight a war against a non-coalition member to get a truce with a coalition member they are allied to. If things fall into place well, you can do a quick and easy war white peace everyone and reset those truces.
(5) dismantling coalitions, part two -- you can attack into a coalition while its small and just white peace them all, this buys you some time for your AE to evaporate
(6) dismantling coalitions, part three -- Let's say you get a typical HRE coalition. It hasn't fired, but you are a bit stuck. Start improving relations, but prioritize larger nations that are farther away from the clay you stole. They will hate you less and drop out more easily. It take 4 nations to build a coalition, but unless some threatening countries are there, a coalition of small-fries rarely triggers.
(7) instead of setting a diplomat (assuming you have a spare) to "outraged countries," try "threatening countries," instead. I find it more helpful to have ok relations with larger non-rivals even if I'm not going to ally them. Your allies care if you ally their rivals, not whether you have OK relations with their rivals. AI nations do not take actions that would de-stabilize themselves.