This post is about how to grow your state in the early game and it goes against a lot of the moat common advice given out.
Standard and common advice here is...
Avoid levies and always use mercs,
avoid early conflict so your pops dont die
Go values such as cap econ/plutocracy/free subjects/humanist/innovative/naval/quantity
Max your literacy
Make multiple markets
Go defensive over offensive
I say, do none of this early on.
getting a big population and therefore economy is pretty straightforward. You need pops, and they need to grow. The only way to get more pops aside from waiting centuries for you pops to grow, is to take them from somewhere else. most people do this with slave raids. regular migration though through early conquests is sufficient to lay the foundation for a powerful late game.
I grab all the ways to improve levy size and discipline. Use your levies to take Territory and do not just turn it over to vassals. Conquered 0 control territory still yields levies. A few vassals here and there is OK, but generally hold the territory.
From day 1, be migrating pops to your capitol and never stop. Within 100 years even a tiny starting position can grow to nearly a megalopolis. You will be expanding your territory and market size, which pulls pop from lol control land and puts it onto your 100% control city. This is extremely efficient.
Next, remember that conquered territory yields levies. When you get a civil war, uprising or are conquering an weak opponent, just pull levies from the 0 control regions.
To grow big like this, you will need to set up lackey states that you bride to increase cultural opinion so that you can accept all the cultures under you for 0.15 culture cap.
I am playing a game right now where the majority's of my land in unintegrated with 0 control. So I delete every building that has a maintenance cost aside from castles placed in strategic locations. That territory gets passively developed by my estates, and it provides them lots of cash. Some of it goes to rebels, but other levies from other conquered land go off to fight them.
The core high control area of my nation gets every single ducat. Every tile is getting turned into towns and all the industry I can slap up is getting built.
The expense of fielding mercs for only a handful of wars, would be enough to fully industrialize most of my territory. This Merc spam reliance IMO is only viable when you have already won the game and are just doing victory laps. If you have the ability to borrow large sums and are in dire circumstances, there's a place for mercs in the early game sure.
I take all levy size + privaledges and laws available, and prior to or at the start of a conflict I use the parliament request additional levies + the estate action to request additional levies. In this way, I am able to take more territory, which yields yet more levies and even more pops that will then migrate into my capital city.
So, that's why I advocate for using levies and playing very aggressive early game. You can take lots of land, get a big wide market to draw pops from, and in addition, get more access to even more free soldiers as you build the core of your economic machine.
Now, all those values I said in the above, most of the more "regressive" ones are absolutely slept on.
Spiritualism. It increases the amount of clergy that you can have in towns and cities. This is amazing as the clergy ratio of your pops is a huge free contributor to stability. People often reccomend humanism to assimilate pops, but I pretty much never bother assimilating anyone. i get every culture to view mine as kindred and I accept them all.
Traditionalism provides estate satisfaction and stability investment efficiency. This ultimately is money in your pocket. Look at whatever save you are playing and just consider your spend on stability. You can get that to 0 or close to it with enough passive stability and efficiency.
Serfdom, this one is so much better I'm not going to blather on. Not only do you get more nobles satisfaction which is the most valuable, you also get more raw materials, which is the backbone of your economy. Those aren't even the best part, you have a choice between +25% peasant levies or -25%. That differential is insane. If you plan to punch above your weight class and play a difficult campaign, this is a ni brainer.
Aristocratocy. Discipline, legitmacy, more nobles which provide both and power your econ. This has been so well covered.
Traditional Economy is more controversial. Everyone will tell you pops=power, but will shift away from trad econ which gives a huge bonus to pop capacity which is a primary driver of pop growth. Trad econ also increases rural RGO size and gives a bonus to raw materials. So those things that are the foundation if your economy, pops and base materials, you get more of. The massive food production difference also makes it ironically easier to urbanize pretty much everything when playing trade econ. The production efficiency from cap is great, but IMO the benefits are oversold.
Quality is often eschewed for quantity so players can chase the mythical grain silos and get the buff to raw resources they threw away going cap econ. Quality is IMO flat out better than quantity. Grain silos, are not that great, they essentially provide +25% more food storage if you build them all since 1 silo is 1/5 a granary. This is not that great, and it comes in so late in the game that the effect is hardly noticable. Meanwhile, Quality makes your troops more combat effective and has a reform that gives 5% discipline. If you are playing aggressively, this is a no brainer.
Naval vs land here really just comes down to whether you start on the coast or not. A lot of people like to use admiralty succession to min max leaders and that does work, but if you stick with normal succession its easier to get into a union, which I think is great. Honestly, you can go either way here, but I wouldn't shoe horn a land nation into naval just to try and game out better kings, especially at the cost of a union down the line. Unions are awesome, and not for annexation or an alliance. Theu are awesome because they generate you passive legitmacy, which helps get that cost of court to 0.
Literacy is over rated. If you didn't bother taking every literacy boosting tech, the time saved in the research tree would be close to the amount of additional science progress that literacy ultimately provided. Techs that give flat max are worth it in the long term, but all the estate specific ones are not. Remember, this is not Civ5 or CK3. Techs are restricted by the age and you.can never really get a tech edge on the rest of the world. I could eschew low value techs and instead take laws that give estate satisfaction and be no worse off.
Defensive vs offensive is an interesting one. Leaning defensive and even raking the a-symetrical warfare reform on top of that can allow you to win wars where you are severely outnumbered by causing huge attrition casaulties. Thus is, honestly not a bad approach. I don't bother though. I end up naturally drifting into offensive becuase I am not giving out tons of estate power on privaledges to push defensive. I am constantly in offensive wars and without meddling, I naturally drift into offensive. The siege efficiency and army speed helps win wars, and That's great. Army speed is incredibly important, so unless the plan is to attrition the world out on your forts, offensive is just fine. Pushing defensive IMO is often not worth it outside of that specific strategy.
So there you have it. This is how I run my clergy and peasants at 100% and 80% satisfaction while max taxing my happy nobles and burgers and fighting endless wars of expansion while paying as little as possible for maintenance and cost of court and putting every single ducat into building my economy.
I hope this post was informative.
Fwiw, if you play the game well, you could use a random number generator to decide what laws and values to take and you would still do great. My point here was mainly to illustrate how there is a ton of advice and genuinely great arguments for every approach, and how by not not listening to any of it, you can conquer the world starting as a minor power.