I see the point in a lot of your criticisms, GAGA Extrem. You're right that in a lot of cases you can more or less do things "instantly" or nearly so in EUIV if you so desire. I guess I haven't found that to be an issue because, at least so far, I've not had any need to and thus haven't. I've by and large played war exhaustion the old way, letting it decline slowly (particularly once I got the Optimism idea), and when Austria was getting hammered and spending diplo points to reduce exhaustion, the effect on their diplomatic tech became noticeable. And while yes, with the economic idea your could take direct action to reduce inflation (and perhaps allowing that isn't a great idea), you still have to pursue that path (idea group) in the first place.
The potential for weakening military via military ideas is an interesting one. I considered that during my game, and in the end I think it's another one of those decisions. At some times, yes, going for the tech will be an obvious choice, but I don't think it's always so clear-cut. Maybe this could be alleviated by getting some bonus just for choosing an idea group. But I don't feel it's really necessary at this point.
So far, I disagree with the lack of long-term planning, however. I think choosing idea groups really can impact long-term planning. As a relatively small country (now mid-sized), I found myself carefully weighing the impact of going for the Diplomatic Ideas versus military ideas, and later whether I could afford Economic Ideas or should perhaps go Defensive, etc. With strong rivals, it mattered. True, my choices were at times impacted by, for example, a surplus of military points, even though as a Republic I rarely had a military-favoring ruler. But there definitely was long-term investment. And I think rebels are more interesting than in EU3. I've seen a lot more cases where there are enough rebels that you either have to accept their demands/let them enforce them, or commit serious resources to fighting them (which may not always be realistic). Do they matter as much as Vicky2 or CKII? Perhaps not, but as someone who's mostly played EU3, they are more potent. I've definitely had significant internal struggle during the Reformation. IMO diplomatic actions also tend to have longer-term effects in EU4 than EU3. Declining a call to arms no longer just means -25 prestige, but -25 prestige and a very difficult time getting that country to realign with you anytime soon, for example. Sliders were long-term in EU3, but as you said were terribly balanced. Unless I was role-playing, I don't know why I would have chosen +1 Serfdom over -1 Stability in EU3, barring my country absolutely falling apart as it was. And maybe this will change as I play EUIV more, but I have occasionally chosen to lose monarch points in events where I didn't have to.
My story has progressed to circa 1648. In the late 1500s I finally overcome Austria, having become stronger from conquering Tuscany and taking advantage of Austria's other wars, and by 1610 my northeast border is peaceful. In the interim, circa 1560, I try going Protestant, and nearly immediately realize the errors of my ways as there's no way I can fight the rebellions. Fortunately I'm able to accept the demands of some Catholic rebels, return to the Church, and limit the damage.
The early 1600s see sparring with Spain, who has reduced France to being less powerful than myself. Though I lose significant armies in the process, in the end Spain leaves too few troops at home, and a coalition of myself, Algiers, the Mamluks, and all their neighbors save Portugal are able to inflict a defeat on them and, for the time being, stop their expansionistic ways in Europe. There also some epic naval battles at Malta and the Balaeres in the process, with the Mamluks and Algerians helping me to duel Spain. The climactic battle saw 56 Spanish ships, with many Galleons, dueling 146 allied ships, primarily Galleys with some small ships, and in the end, the destruction of the Spanish fleet that we'd battled off and on for several years. The Armada was defeated, and not too long afterwards Spain would be defeated.
Around 1632 I had a large Protestant uprising, comprising some 65,000 troops - more than double the size of my standing army. Although I likely could have defeated them, it would have been quite costly, so in the end I decided to let them enforce their demands (saving myself -50 prestige), and try to make the -10% idea cost and +10% taxes make up for conversion. By 1638, I was seriously questioning this move, and quite nearly went back to Catholicism again when I resumed the game. However, I now believe that everything will work out in the end. And if it doesn't, well, we can always try going back to Catholicism...
Algiers is my staunchest ally currently, having been allied for about 80 years (including 60 before the alliance auto-broke when I changed religions - I'm not sure why a Sunni country should really care, but they renewed the alliance right away, so no significant harm). Bremen is my new Protestant ally. But perhaps my most interesting ally is Mali, who I initially allied with after the Spanish war to discourage Spain from fighting them again (Mali had actually made a contribution in that war - they had over 90,000 rebels during the war, who bottled up 20,000 Spanish troops deep in Malinese territory). Just a few months ago, Mali started westernizing. I'm not sure quite what caused them to do so, but I'm certainly hopeful that it will help them resist the Spanish in the future, and have sent an army over to help them against the reactionaries. What started as an alliance where I didn't expect to get much in return may end up working out both ways after all. It's also noteworthy since I pretty much never saw AI westernization in EU3.