Crusader Kings 3 plays in a much earlier time period, and spans up to 6 centuries. And while the world generally got richer from 867 to 1453, the historic economic growth was a lot slower than that of the industrial revolution. If CK3 had an economic system in which you could make an investment with a reasonable yield, a player-controlled and optimized economy would become incredibly rich over the centuries. Which would be both unbalancing and not historically correct.

So the devs decided to make the economic system of CK3 incredibly low yield. The best investment opportunity is the first level of building up your castles, churches, and cities, and that brings a yield of only 4%. It takes 25 years to just get your initial investment back. And further levels of investment have much less yield, with the higher levels needing centuries to just break even.

If I offered you an investment opportunity that breaks even in a century, you probably wouldn't be interested. Even in the context of a game, and even if you take a long view, the economic gameplay of Crusader Kings 3 isn't very attractive. And it doesn't take much economic genius to discover that building "wide", that is investing in level I buildings in as many counties as possible, is much better than playing "tall", investing in higher level buildings in few counties. That is a bit of a bummer, because it only reinforces the already existing push towards an expansionist game. Even the tutorial wants you to start with a county in Ireland and end up becoming the ruler of the whole island.

Personally, I am not much interested in an expansionist campaign in CK3. I find the warfare system highly annoying, as the AI is programmed to avoid unfavorable battles, and the game is lacking means to somehow force combat or trap the enemy armies. Thus a war feels a lot like chasing a group of chickens, with lots of running around in all directions, and very little actual combat action. If you want to play Crusader Kings 3 as a 4X game, you'll also run into the problem of succession. In the early to mid-game you simply don't have the technology to leave your kingdom to your eldest son, but instead it is divided up between your children (or just your sons). You have very little control about how many children you'll have, with too few risking your dynasty to die out, and too many risking your lands to be spread between too many people.

So I think I will stick with my plan to play a non-expansionist count. But I will "cheat" to start the game with what I'll roleplay as a lucky large inheritance, giving me the money to build the level I buildings quickly. That won't fix the problem of lack of good investments, but at least it will allow me to play with a slightly higher income than smaller counts usually have. With a lot of possible player actions requiring money, playing somebody very poor would mean not having a lot of things to do.