the best way to learn eu4 imo is to play as a japanese daimyou
you're on more or less equal footing with all your neighbours and very insulated from the rest of asia, allowing you to really focus on your goal of uniting japan
portugal, england, and norway aren't bad either, if you pick up exploration ideas and want to learn how colonisation works
ottomans and castile are go-to strong nations, but they're so big and powerful you won't really learn much and can honestly ignore a lot of mechanics like trade and aggressive expansion
Portugal and England are great for learning; England is safe from BBB and it's hard for mistakes to cripple it given that there are so few nations capable of taking advantage of its weakness, whilst it's in prime location for players to learn trade mechanics, naval combat, combat, colonization, espionage, diplomacy, religion and parliamentary/estate mechanics. All the while it has to contend with powerful rivals in Castile/Spain, France and the Netherlands, which while cannot destroy England easily, can still destroy most of its endeavours, providing a challenge. Portugal requires a little more skill imo given their much smaller size but they're also fairly safe given that Castile/Spain protects them well, though there's little more nightmarish for a new player in Portugal to find themselves stuck in a personal union with Spain. And it WILL happen eventually.
IMO Muscovy/Russia is the best one for new players, since Muscovy/Russia is the only major nation that heavily utilizes every single game mechanic - and it doesn't dump them all on the player, they need to be fully exploited as time progresses. Muscovy starts off needing to utilize rivalry and diplomacy to diplomatically unify the smaller Russian states and militarily isolate/conquer Novgorod, and will eventually need a Baltic fleet to ward off Teutonic and Swedish navies. As it expands southwards and eastwards it will need to utilize religious and culture mechanics, need to check its aggressive expansion if it goes westwards, and as Byzantium falls it needs to utilize religious mechanics like defender of the faith to protect the balkan orthodox states from Austrian and Turkish rivals. Capable of expanding in all directions (except north) Russia has plenty of options for new players to pick from, owing to its gargantuan size its hard for Russia to enter a death spiral even if every major power declares war on it, its large army allows for mistakes to be made, it utilizes both colonization and later on colonial nations at Alaska and eventually forms its own massive trade network, it uses naval mechanics in the Med and Baltic and has the best estate mechanics with cossacks. Last but not least it utilizes westernization, but is in the eastern tech group so doesn't really need to westernize if the newby is scared too much of the mechanic
I've gotten advertisements* for dresses, and been like "Ooh, this is pretty, I want - wait, I don't wear dresses. Maybe get a kilt for summer, me?"
* email, like, from Amazon
Haha that reminds me of how my adverts all used to be islamic until one day I kept researching fashion terminology, textiles terminology and processes and fashion examples in order to better explain and understand the significance of a 200 year old silk-damask dress. By the time I was done all of my adverts switched to those expectant of a bride shopping for her wedding