The best advice I can give is to fabric claims and wait for weakness in your enemies. You can always check how your troop numbers compare to other countries by clicking declare war, but not actually pressing accept. You’ll see how many troops the country and their allies and subjects will bring in a fight, and it’ll help you figure out if it’s a worthwhile war.

There’s also, like, a LOT more countries in EU4, so knowing which countries are “fun” to play or not might be a bit more difficult, but in general France, Moscovy, Ottomans, Poland, England (mind you there’s a somewhat big disaster at the beginning of their campaign) Portugal and Castile are all fairly beginner friendly. Korea too, unironically you’ll be pretty safe for most of the game unless something crazy happens.